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git-svn-id: http://htmlpurifier.org/svnroot/htmlpurifier/trunk@1253 48356398-32a2-884e-a903-53898d9a118a
1047 lines
50 KiB
HTML
1047 lines
50 KiB
HTML
<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
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"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
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<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en"><head>
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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
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<meta name="description" content="Describes the rationale for using UTF-8, the ramifications otherwise, and how to make the switch." />
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<link rel="stylesheet" type="text/css" href="./style.css" />
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<script defer="defer" type="text/javascript" src="./toc-gen.js"></script>
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<style type="text/css">
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.minor td {font-style:italic;}
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</style>
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<title>UTF-8: The Secret of Character Encoding - HTML Purifier</title>
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<!-- Note to users: this document, though professing to be UTF-8, attempts
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to use only ASCII characters, because most webservers are configured
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to send HTML as ISO-8859-1. So I will, many times, go against my
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own advice for sake of portability. -->
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</head><body>
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<h1>UTF-8: The Secret of Character Encoding</h1>
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<div id="filing">Filed under End-User</div>
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<div id="index">Return to the <a href="index.html">index</a>.</div>
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<div id="home"><a href="http://htmlpurifier.org/">HTML Purifier</a> End-User Documentation</div>
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<p>Character encoding and character sets are not that
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difficult to understand, but so many people blithely stumble
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through the worlds of programming without knowing what to actually
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do about it, or say "Ah, it's a job for those <em>internationalization</em>
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experts." No, it is not! This document will walk you through
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determining the encoding of your system and how you should handle
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this information. It will stay away from excessive discussion on
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the internals of character encoding.</p>
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<p>This document is not designed to be read in its entirety: it will
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slowly introduce concepts that build on each other: you need not get to
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the bottom to have learned something new. However, I strongly
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recommend you read all the way to <strong>Why UTF-8?</strong>, because at least
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at that point you'd have made a conscious decision not to migrate,
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which can be a rewarding (but difficult) task.</p>
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<blockquote class="aside">
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<div class="label">Asides</div>
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<p>Text in this formatting is an <strong>aside</strong>,
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interesting tidbits for the curious but not strictly necessary material to
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do the tutorial. If you read this text, you'll come out
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with a greater understanding of the underlying issues.</p>
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</blockquote>
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<h2>Table of Contents</h2>
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<ol id="toc">
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<li><a href="#findcharset">Finding the real encoding</a></li>
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<li><a href="#findmetacharset">Finding the embedded encoding</a></li>
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<li><a href="#fixcharset">Fixing the encoding</a><ol>
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<li><a href="#fixcharset-none">No embedded encoding</a></li>
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<li><a href="#fixcharset-diff">Embedded encoding disagrees</a></li>
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<li><a href="#fixcharset-server">Changing the server encoding</a><ol>
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<li><a href="#fixcharset-server-php">PHP header() function</a></li>
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<li><a href="#fixcharset-server-phpini">PHP ini directive</a></li>
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<li><a href="#fixcharset-server-nophp">Non-PHP</a></li>
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<li><a href="#fixcharset-server-htaccess">.htaccess</a></li>
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<li><a href="#fixcharset-server-ext">File extensions</a></li>
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</ol></li>
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<li><a href="#fixcharset-xml">XML</a></li>
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<li><a href="#fixcharset-internals">Inside the process</a></li>
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</ol></li>
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<li><a href="#whyutf8">Why UTF-8?</a><ol>
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<li><a href="#whyutf8-i18n">Internationalization</a></li>
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<li><a href="#whyutf8-user">User-friendly</a></li>
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<li><a href="#whyutf8-forms">Forms</a><ol>
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<li><a href="#whyutf8-forms-urlencoded">application/x-www-form-urlencoded</a></li>
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<li><a href="#whyutf8-forms-multipart">multipart/form-data</a></li>
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</ol></li>
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<li><a href="#whyutf8-support">Well supported</a></li>
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<li><a href="#whyutf8-htmlpurifier">HTML Purifiers</a></li>
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</ol></li>
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<li><a href="#migrate">Migrate to UTF-8</a><ol>
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<li><a href="#migrate-db">Configuring your database</a><ol>
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<li><a href="#migrate-db-legit">Legit method</a></li>
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<li><a href="#migrate-db-binary">Binary</a></li>
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</ol></li>
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<li><a href="#migrate-editor">Text editor</a></li>
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<li><a href="#migrate-bom">Byte Order Mark (headers already sent!)</a></li>
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<li><a href="#migrate-fonts">Fonts</a><ol>
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<li><a href="#migrate-fonts-obscure">Obscure scripts</a></li>
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<li><a href="#migrate-fonts-occasional">Occasional use</a></li>
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</ol></li>
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<li><a href="#migrate-variablewidth">Dealing with variable width in functions</a></li>
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</ol></li>
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<li><a href="#externallinks">Further Reading</a></li>
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</ol>
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<h2 id="findcharset">Finding the real encoding</h2>
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<p>In the beginning, there was ASCII, and things were simple. But they
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weren't good, for no one could write in Cryllic or Thai. So there
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exploded a proliferation of character encodings to remedy the problem
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by extending the characters ASCII could express. This ridiculously
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simplified version of the history of character encodings shows us that
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there are now many character encodings floating around.</p>
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<blockquote class="aside">
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<p>A <strong>character encoding</strong> tells the computer how to
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interpret raw zeroes and ones into real characters. It
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usually does this by pairing numbers with characters.</p>
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<p>There are many different types of character encodings floating
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around, but the ones we deal most frequently with are ASCII,
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8-bit encodings, and Unicode-based encodings.</p>
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<ul>
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<li><strong>ASCII</strong> is a 7-bit encoding based on the
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English alphabet.</li>
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<li><strong>8-bit encodings</strong> are extensions to ASCII
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that add a potpourri of useful, non-standard characters
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like é and æ. They can only add 127 characters,
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so usually only support one script at a time. When you
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see a page on the web, chances are it's encoded in one
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of these encodings.</li>
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<li><strong>Unicode-based encodings</strong> implement the
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Unicode standard and include UTF-8, UCS-2 and UTF-16.
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They go beyond 8-bits (the first two are variable length,
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while the second one uses 16-bits), and support almost
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every language in the world. UTF-8 is gaining traction
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as the dominant international encoding of the web.</li>
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</ul>
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</blockquote>
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<p>The first step of our journey is to find out what the encoding of
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your website is. The most reliable way is to ask your
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browser:</p>
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<dl>
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<dt>Mozilla Firefox</dt>
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<dd>Tools > Page Info: Encoding</dd>
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<dt>Internet Explorer</dt>
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<dd>View > Encoding: bulleted item is unofficial name</dd>
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</dl>
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<p>Internet Explorer won't give you the mime (i.e. useful/real) name of the
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character encoding, so you'll have to look it up using their description.
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Some common ones:</p>
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<table class="table">
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<thead><tr>
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<th>IE's Description</th>
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<th>Mime Name</th>
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</tr></thead>
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<tbody>
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<tr><th colspan="2">Windows</th></tr>
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<tr><td>Arabic (Windows)</td><td>Windows-1256</td></tr>
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<tr><td>Baltic (Windows)</td><td>Windows-1257</td></tr>
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<tr><td>Central European (Windows)</td><td>Windows-1250</td></tr>
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<tr><td>Cyrillic (Windows)</td><td>Windows-1251</td></tr>
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<tr><td>Greek (Windows)</td><td>Windows-1253</td></tr>
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<tr><td>Hebrew (Windows)</td><td>Windows-1255</td></tr>
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<tr><td>Thai (Windows)</td><td>TIS-620</td></tr>
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<tr><td>Turkish (Windows)</td><td>Windows-1254</td></tr>
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<tr><td>Vietnamese (Windows)</td><td>Windows-1258</td></tr>
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<tr><td>Western European (Windows)</td><td>Windows-1252</td></tr>
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</tbody>
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<tbody>
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<tr><th colspan="2">ISO</th></tr>
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<tr><td>Arabic (ISO)</td><td>ISO-8859-6</td></tr>
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<tr><td>Baltic (ISO)</td><td>ISO-8859-4</td></tr>
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<tr><td>Central European (ISO)</td><td>ISO-8859-2</td></tr>
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<tr><td>Cyrillic (ISO)</td><td>ISO-8859-5</td></tr>
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<tr class="minor"><td>Estonian (ISO)</td><td>ISO-8859-13</td></tr>
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<tr class="minor"><td>Greek (ISO)</td><td>ISO-8859-7</td></tr>
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<tr><td>Hebrew (ISO-Logical)</td><td>ISO-8859-8-l</td></tr>
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<tr><td>Hebrew (ISO-Visual)</td><td>ISO-8859-8</td></tr>
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<tr class="minor"><td>Latin 9 (ISO)</td><td>ISO-8859-15</td></tr>
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<tr class="minor"><td>Turkish (ISO)</td><td>ISO-8859-9</td></tr>
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<tr><td>Western European (ISO)</td><td>ISO-8859-1</td></tr>
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</tbody>
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<tbody>
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<tr><th colspan="2">Other</th></tr>
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<tr><td>Chinese Simplified (GB18030)</td><td>GB18030</td></tr>
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<tr><td>Chinese Simplified (GB2312)</td><td>GB2312</td></tr>
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<tr><td>Chinese Simplified (HZ)</td><td>HZ</td></tr>
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<tr><td>Chinese Traditional (Big5)</td><td>Big5</td></tr>
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<tr><td>Japanese (Shift-JIS)</td><td>Shift_JIS</td></tr>
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<tr><td>Japanese (EUC)</td><td>EUC-JP</td></tr>
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<tr><td>Korean</td><td>EUC-KR</td></tr>
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<tr><td>Unicode (UTF-8)</td><td>UTF-8</td></tr>
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</tbody>
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</table>
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<p>Internet Explorer does not recognize some of the more obscure
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character encodings, and having to lookup the real names with a table
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is a pain, so I recommend using Mozilla Firefox to find out your
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character encoding.</p>
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<h2 id="findmetacharset">Finding the embedded encoding</h2>
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<p>At this point, you may be asking, "Didn't we already find out our
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encoding?" Well, as it turns out, there are multiple places where
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a web developer can specify a character encoding, and one such place
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is in a <code>META</code> tag:</p>
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<pre><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /></pre>
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<p>You'll find this in the <code>HEAD</code> section of an HTML document.
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The text to the right of <code>charset=</code> is the "claimed"
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encoding: the HTML claims to be this encoding, but whether or not this
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is actually the case depends on other factors. For now, take note
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if your <code>META</code> tag claims that either:</p>
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<ol>
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<li>The character encoding is the same as the one reported by the
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browser,</li>
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<li>The character encoding is different from the browser's, or</li>
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<li>There is no <code>META</code> tag at all! (horror, horror!)</li>
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</ol>
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<h2 id="fixcharset">Fixing the encoding</h2>
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<p>If your <code>META</code> encoding and your real encoding match,
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savvy! You can skip this section. If they don't...</p>
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<h3 id="fixcharset-none">No embedded encoding</h3>
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<p>If this is the case, you'll want to add in the appropriate
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<code>META</code> tag to your website. It's as simple as copy-pasting
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the code snippet above and replacing UTF-8 with whatever is the mime name
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of your real encoding.</p>
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<blockquote class="aside">
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<p>For all those skeptics out there, there is a very good reason
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why the character encoding should be explicitly stated. When the
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browser isn't told what the character encoding of a text is, it
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has to guess: and sometimes the guess is wrong. Hackers can manipulate
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this guess in order to slip XSS pass filters and then fool the
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browser into executing it as active code. A great example of this
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is the <a href="http://shiflett.org/archive/177">Google UTF-7
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exploit</a>.</p>
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<p>You might be able to get away with not specifying a character
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encoding with the <code>META</code> tag as long as your webserver
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sends the right Content-Type header, but why risk it? Besides, if
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the user downloads the HTML file, there is no longer any webserver
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to define the character encoding.</p>
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</blockquote>
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<h3 id="fixcharset-diff">Embedded encoding disagrees</h3>
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<p>This is an extremely common mistake: another source is telling
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the browser what the
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character encoding is and is overriding the embedded encoding. This
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source usually is the Content-Type HTTP header that the webserver (i.e.
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Apache) sends. A usual Content-Type header sent with a page might
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look like this:</p>
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<pre>Content-Type: text/html; charset=ISO-8859-1</pre>
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<p>Notice how there is a charset parameter: this is the webserver's
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way of telling a browser what the character encoding is, much like
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the <code>META</code> tags we touched upon previously.</p>
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<blockquote class="aside"><p>In fact, the <code>META</code> tag is
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designed as a substitute for the HTTP header for contexts where
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sending headers is impossible (such as locally stored files without
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a webserver). Thus the name <code>http-equiv</code> (HTTP equivalent).
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</p></blockquote>
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<p>There are two ways to go about fixing this: changing the <code>META</code>
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tag to match the HTTP header, or changing the HTTP header to match
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the <code>META</code> tag. How do we know which to do? It depends
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on the website's content: after all, headers and tags are only ways of
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describing the actual characters on the web page.</p>
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<p>If your website:</p>
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<dl>
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<dt>...only uses ASCII characters,</dt>
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<dd>Either way is fine, but I recommend switching both to
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UTF-8 (more on this later).</dd>
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<dt>...uses special characters, and they display
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properly,</dt>
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<dd>Change the embedded encoding to the server encoding.</dd>
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<dt>...uses special characters, but users often complain that
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they come out garbled,</dt>
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<dd>Change the server encoding to the embedded encoding.</dd>
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</dl>
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<p>Changing a META tag is easy: just swap out the old encoding
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for the new. Changing the server (HTTP header) encoding, however,
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is slightly more difficult.</p>
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<h3 id="fixcharset-server">Changing the server encoding</h3>
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<h4 id="fixcharset-server-php">PHP header() function</h4>
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<p>The simplest way to handle this problem is to send the encoding
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yourself, via your programming language. Since you're using HTML
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Purifier, I'll assume PHP, although it's not too difficult to do
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similar things in
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<a href="http://www.w3.org/International/O-HTTP-charset#scripting">other
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languages</a>. The appropriate code is:</p>
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<pre><a href="http://php.net/function.header">header</a>('Content-Type:text/html; charset=UTF-8');</pre>
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<p>...replacing UTF-8 with whatever your embedded encoding is.
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This code must come before any output, so be careful about
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stray whitespace in your application.</p>
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<h4 id="fixcharset-server-phpini">PHP ini directive</h4>
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<p>PHP also has a neat little ini directive that can save you a
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header call: <code><a href="http://php.net/ini.core#ini.default-charset">default_charset</a></code>. Using this code:</p>
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<pre><a href="http://php.net/function.ini_set">ini_set</a>('default_charset', 'UTF-8');</pre>
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<p>...will also do the trick. If PHP is running as an Apache module (and
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not as FastCGI, consult
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<a href="http://php.net/phpinfo">phpinfo</a>() for details), you can even use htaccess do apply this property
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globally:</p>
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<pre><a href="http://php.net/configuration.changes#configuration.changes.apache">php_value</a> default_charset "UTF-8"</pre>
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<blockquote class="aside"><p>As with all INI directives, this can
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also go in your php.ini file. Some hosting providers allow you to customize
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your own php.ini file, ask your support for details. Use:</p>
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<pre>default_charset = "utf-8"</pre></blockquote>
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<h4 id="fixcharset-server-nophp">Non-PHP</h4>
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<p>You may, for whatever reason, need to set the character encoding
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on non-PHP files, usually plain ol' HTML files. Doing this
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is more of a hit-or-miss process: depending on the software being
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used as a webserver and the configuration of that software, certain
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techniques may work, or may not work.</p>
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<h4 id="fixcharset-server-htaccess">.htaccess</h4>
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<p>On Apache, you can use an .htaccess file to change the character
|
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encoding. I'll defer to
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<a href="http://www.w3.org/International/questions/qa-htaccess-charset">W3C</a>
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for the in-depth explanation, but it boils down to creating a file
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named .htaccess with the contents:</p>
|
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|
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<pre><a href="http://httpd.apache.org/docs/1.3/mod/mod_mime.html#addcharset">AddCharset</a> UTF-8 .html</pre>
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|
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<p>Where UTF-8 is replaced with the character encoding you want to
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use and .html is a file extension that this will be applied to. This
|
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character encoding will then be set for any file directly in
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or in the subdirectories of directory you place this file in.</p>
|
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|
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<p>If you're feeling particularly courageous, you can use:</p>
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<pre><a href="http://httpd.apache.org/docs/1.3/mod/core.html#adddefaultcharset">AddDefaultCharset</a> UTF-8</pre>
|
|
|
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<p>...which changes the character set Apache adds to any document that
|
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doesn't have any Content-Type parameters. This directive, which the
|
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default configuration file sets to iso-8859-1 for security
|
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reasons, is probably why your headers mismatch
|
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with the <code>META</code> tag. If you would prefer Apache not to be
|
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butting in on your character encodings, you can tell it not
|
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to send anything at all:</p>
|
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|
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<pre><a href="http://httpd.apache.org/docs/1.3/mod/core.html#adddefaultcharset">AddDefaultCharset</a> Off</pre>
|
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|
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<p>...making your <code>META</code> tags the sole source of
|
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character encoding information. In these cases, it is
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<em>especially</em> important to make sure you have valid <code>META</code>
|
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tags on your pages and all the text before them is ASCII.</p>
|
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|
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<blockquote class="aside"><p>These directives can also be
|
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placed in httpd.conf file for Apache, but
|
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in most shared hosting situations you won't be able to edit this file.
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</p></blockquote>
|
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|
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<h4 id="fixcharset-server-ext">File extensions</h4>
|
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|
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<p>If you're not allowed to use .htaccess files, you can often
|
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piggy-back off of Apache's default AddCharset declarations to get
|
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your files in the proper extension. Here are Apache's default
|
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character set declarations:</p>
|
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|
|
<table class="table">
|
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<thead><tr>
|
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<th>Charset</th>
|
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<th>File extension(s)</th>
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</tr></thead>
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<tbody>
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<tr><td>ISO-8859-1</td><td>.iso8859-1 .latin1</td></tr>
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<tr><td>ISO-8859-2</td><td>.iso8859-2 .latin2 .cen</td></tr>
|
|
<tr><td>ISO-8859-3</td><td>.iso8859-3 .latin3</td></tr>
|
|
<tr><td>ISO-8859-4</td><td>.iso8859-4 .latin4</td></tr>
|
|
<tr><td>ISO-8859-5</td><td>.iso8859-5 .latin5 .cyr .iso-ru</td></tr>
|
|
<tr><td>ISO-8859-6</td><td>.iso8859-6 .latin6 .arb</td></tr>
|
|
<tr><td>ISO-8859-7</td><td>.iso8859-7 .latin7 .grk</td></tr>
|
|
<tr><td>ISO-8859-8</td><td>.iso8859-8 .latin8 .heb</td></tr>
|
|
<tr><td>ISO-8859-9</td><td>.iso8859-9 .latin9 .trk</td></tr>
|
|
<tr><td>ISO-2022-JP</td><td>.iso2022-jp .jis</td></tr>
|
|
<tr><td>ISO-2022-KR</td><td>.iso2022-kr .kis</td></tr>
|
|
<tr><td>ISO-2022-CN</td><td>.iso2022-cn .cis</td></tr>
|
|
<tr><td>Big5</td><td>.Big5 .big5 .b5</td></tr>
|
|
<tr><td>WINDOWS-1251</td><td>.cp-1251 .win-1251</td></tr>
|
|
<tr><td>CP866</td><td>.cp866</td></tr>
|
|
<tr><td>KOI8-r</td><td>.koi8-r .koi8-ru</td></tr>
|
|
<tr><td>KOI8-ru</td><td>.koi8-uk .ua</td></tr>
|
|
<tr><td>ISO-10646-UCS-2</td><td>.ucs2</td></tr>
|
|
<tr><td>ISO-10646-UCS-4</td><td>.ucs4</td></tr>
|
|
<tr><td>UTF-8</td><td>.utf8</td></tr>
|
|
<tr><td>GB2312</td><td>.gb2312 .gb </td></tr>
|
|
<tr><td>utf-7</td><td>.utf7</td></tr>
|
|
<tr><td>EUC-TW</td><td>.euc-tw</td></tr>
|
|
<tr><td>EUC-JP</td><td>.euc-jp</td></tr>
|
|
<tr><td>EUC-KR</td><td>.euc-kr</td></tr>
|
|
<tr><td>shift_jis</td><td>.sjis</td></tr>
|
|
</tbody>
|
|
</table>
|
|
|
|
<p>So, for example, a file named <code>page.utf8.html</code> or
|
|
<code>page.html.utf8</code> will probably be sent with the UTF-8 charset
|
|
attached, the difference being that if there is an
|
|
<code>AddCharset charset .html</code> declaration, it will override
|
|
the .utf8 extension in <code>page.utf8.html</code> (precedence moves
|
|
from right to left). By default, Apache has no such declaration.</p>
|
|
|
|
<h4 id="fixcharset-server-iis">Microsoft IIS</h4>
|
|
|
|
<p>If anyone can contribute information on how to configure Microsoft
|
|
IIS to change character encodings, I'd be grateful.</p>
|
|
|
|
<h3 id="fixcharset-xml">XML</h3>
|
|
|
|
<p><code>META</code> tags are the most common source of embedded
|
|
encodings, but they can also come from somewhere else: XML
|
|
processing instructions. They look like:</p>
|
|
|
|
<pre><?xml version="1.0" encoding="UTF-8"?></pre>
|
|
|
|
<p>...and are most often found in XML documents (including XHTML).</p>
|
|
|
|
<p>For XHTML, this processing instruction theoretically
|
|
overrides the <code>META</code> tag. In reality, this happens only when the
|
|
XHTML is actually served as legit XML and not HTML, which is almost always
|
|
never due to Internet Explorer's lack of support for
|
|
<code>application/xhtml+xml</code> (even though doing so is often
|
|
argued to be <a href="http://www.hixie.ch/advocacy/xhtml">good practice</a>).</p>
|
|
|
|
<p>For XML, however, this processing instruction is extremely important.
|
|
Since most webservers are not configured to send charsets for .xml files,
|
|
this is the only thing a parser has to go on. Furthermore, the default
|
|
for XML files is UTF-8, which often butts heads with more common
|
|
ISO-8859-1 encoding (you see this in garbled RSS feeds).</p>
|
|
|
|
<p>In short, if you use XHTML and have gone through the
|
|
trouble of adding the XML header, make sure it jives
|
|
with your <code>META</code> tags and HTTP headers.</p>
|
|
|
|
<h3 id="fixcharset-internals">Inside the process</h3>
|
|
|
|
<p>This section is not required reading,
|
|
but may answer some of your questions on what's going on in all
|
|
this character encoding hocus pocus. If you're interested in
|
|
moving on to the next phase, skip this section.</p>
|
|
|
|
<p>A logical question that follows all of our wheeling and dealing
|
|
with multiple sources of character encodings is "Why are there
|
|
so many options?" To answer this question, we have to turn
|
|
back our definition of character encodings: they allow a program
|
|
to interpret bytes into human-readable characters.</p>
|
|
|
|
<p>Thus, a chicken-egg problem: a character encoding
|
|
is necessary to interpret the
|
|
text of a document. A <code>META</code> tag is in the text of a document.
|
|
The <code>META</code> tag gives the character encoding. How can we
|
|
determine the contents of a <code>META</code> tag, inside the text,
|
|
if we don't know it's character encoding? And how do we figure out
|
|
the character encoding, if we don't know the contents of the
|
|
<code>META</code> tag?</p>
|
|
|
|
<p>Fortunantely for us, the characters we need to write the
|
|
<code>META</code> are in ASCII, which is pretty much universal
|
|
over every character encoding that is in common use today. So,
|
|
all the web-browser has to do is parse all the way down until
|
|
it gets to the Content-Type tag, extract the character encoding
|
|
tag, then re-parse the document according to this new information.</p>
|
|
|
|
<p>Obviously this is complicated, so browsers prefer the simpler
|
|
and more efficient solution: get the character encoding from a
|
|
somewhere other than the document itself, i.e. the HTTP headers,
|
|
much to the chagrin of HTML authors who can't set these headers.</p>
|
|
|
|
<h2 id="whyutf8">Why UTF-8?</h2>
|
|
|
|
<p>So, you've gone through all the trouble of ensuring that your
|
|
server and embedded characters all line up properly and are
|
|
present. Good job: at
|
|
this point, you could quit and rest easy knowing that your pages
|
|
are not vulnerable to character encoding style XSS attacks.
|
|
However, just as having a character encoding is better than
|
|
having no character encoding at all, having UTF-8 as your
|
|
character encoding is better than having some other random
|
|
character encoding, and the next step is to convert to UTF-8.
|
|
But why?</p>
|
|
|
|
<h3 id="whyutf8-i18n">Internationalization</h3>
|
|
|
|
<p>Many software projects, at one point or another, suddenly realize
|
|
that they should be supporting more than one language. Even regular
|
|
usage in one language sometimes requires the occasional special character
|
|
that, without surprise, is not available in your character set. Sometimes
|
|
developers get around this by adding support for multiple encodings: when
|
|
using Chinese, use Big5, when using Japanese, use Shift-JIS, when
|
|
using Greek, etc. Other times, they use character entities with great
|
|
zeal.</p>
|
|
|
|
<p>UTF-8, however, obviates the need for any of these complicated
|
|
measures. After getting the system to use UTF-8 and adjusting for
|
|
sources that are outside the hand of the browser (more on this later),
|
|
UTF-8 just works. You can use it for any language, even many languages
|
|
at once, you don't have to worry about managing multiple encodings,
|
|
you don't have to use those user-unfriendly entities.</p>
|
|
|
|
<h3 id="whyutf8-user">User-friendly</h3>
|
|
|
|
<p>Websites encoded in Latin-1 (ISO-8859-1) which ocassionally need
|
|
a special character outside of their scope often will use a character
|
|
entity to achieve the desired effect. For instance, θ can be
|
|
written <code>&theta;</code>, regardless of the character encoding's
|
|
support of Greek letters.</p>
|
|
|
|
<p>This works nicely for limited use of special characters, but
|
|
say you wanted this sentence of Chinese text: 激光,
|
|
這兩個字是甚麼意思.
|
|
The entity-ized version would look like this:</p>
|
|
|
|
<pre>&#28608;&#20809;, &#36889;&#20841;&#20491;&#23383;&#26159;&#29978;&#40636;&#24847;&#24605;</pre>
|
|
|
|
<p>Extremely inconvenient for those of us who actually know what
|
|
character entities are, totally unintelligible to poor users who don't!
|
|
Even the slightly more user-friendly, "intelligible" character
|
|
entities like <code>&theta;</code> will leave users who are
|
|
uninterested in learning HTML scratching their heads. On the other
|
|
hand, if they see θ in an edit box, they'll know that it's a
|
|
special character, and treat it accordingly, even if they don't know
|
|
how to write that character themselves.</p>
|
|
|
|
<blockquote class="aside"><p>Wikipedia is a great case study for
|
|
an application that originally used ISO-8859-1 but switched to UTF-8
|
|
when it became far to cumbersome to support foreign languages. Bots
|
|
will now actually go through articles and convert character entities
|
|
to their corresponding real characters for the sake of user-friendliness
|
|
and searcheability. See
|
|
<a href="http://meta.wikimedia.org/wiki/Help:Special_characters">Meta's
|
|
page on special characters</a> for more details.
|
|
</p></blockquote>
|
|
|
|
<h3 id="whyutf8-forms">Forms</h3>
|
|
|
|
<p>While we're on the tack of users, how do non-UTF-8 web forms deal
|
|
with characters that our outside of their character set? Rather than
|
|
discuss what UTF-8 does right, we're going to show what could go wrong
|
|
if you didn't use UTF-8 and people tried to use characters outside
|
|
of your character encoding.</p>
|
|
|
|
<p>The troubles are large, extensive, and extremely difficult to fix (or,
|
|
at least, difficult enough that if you had the time and resources to invest
|
|
in doing the fix, you would be probably better off migrating to UTF-8).
|
|
There are two types of form submission: <code>application/x-www-form-urlencoded</code>
|
|
which is used for GET and by default for POST, and <code>multipart/form-data</code>
|
|
which may be used by POST, and is required when you want to upload
|
|
files.</p>
|
|
|
|
<p>The following is a summarization of notes from
|
|
<a href="http://ppewww.physics.gla.ac.uk/~flavell/charset/form-i18n.html">
|
|
<code>FORM</code> submission and i18n</a>. That document contains lots
|
|
of useful information, but is written in a rambly manner, so
|
|
here I try to get right to the point.</p>
|
|
|
|
<h4 id="whyutf8-forms-urlencoded"><code>application/x-www-form-urlencoded</code></h4>
|
|
|
|
<p>This is the Content-Type that GET requests must use, and POST requests
|
|
use by default. It involves the ubiquituous percent encoding format that
|
|
looks something like: <code>%C3%86</code>. There is no official way of
|
|
determining the character encoding of such a request, since the percent
|
|
encoding operates on a byte level, so it is usually assumed that it
|
|
is the same as the encoding the page containing the form was submitted
|
|
in. You'll run into very few problems if you only use characters in
|
|
the character encoding you chose.</p>
|
|
|
|
<p>However, once you start adding characters outside of your encoding
|
|
(and this is a lot more common than you may think: take curly
|
|
"smart" quotes from Microsoft as an example),
|
|
a whole manner of strange things start to happen. Depending on the
|
|
browser you're using, they might:</p>
|
|
|
|
<ul>
|
|
<li>Replace the unsupported characters with useless question marks,</li>
|
|
<li>Attempt to fix the characters (example: smart quotes to regular quotes),</li>
|
|
<li>Replace the character with a character entity, or</li>
|
|
<li>Send it anyway as a different character encoding mixed in
|
|
with the original encoding (usually Windows-1252 rather than
|
|
iso-8859-1 or UTF-8 interspersed in 8-bit)</li>
|
|
</ul>
|
|
|
|
<p>To properly guard against these behaviors, you'd have to sniff out
|
|
the browser agent, compile a database of different behaviors, and
|
|
take appropriate conversion action against the string (disregarding
|
|
a spate of extremely mysterious, random and devastating bugs Internet
|
|
Explorer manifests every once in a while). Or you could
|
|
use UTF-8 and rest easy knowing that none of this could possibly happen
|
|
since UTF-8 supports every character.</p>
|
|
|
|
<h4 id="whyutf8-forms-multipart"><code>multipart/form-data</code></h4>
|
|
|
|
<p>Multipart form submission takes a way a lot of the ambiguity
|
|
that percent-encoding had: the server now can explicitly ask for
|
|
certain encodings, and the client can explicitly tell the server
|
|
during the form submission what encoding the fields are in.</p>
|
|
|
|
<p>There are two ways you go with this functionality: leave it
|
|
unset and have the browser send in the same encoding as the page,
|
|
or set it to UTF-8 and then do another conversion server-side.
|
|
Each method has deficiencies, especially the former.</p>
|
|
|
|
<p>If you tell the browser to send the form in the same encoding as
|
|
the page, you still have the trouble of what to do with characters
|
|
that are outside of the character encoding's range. The behavior, once
|
|
again, varies: Firefox 2.0 entity-izes them while Internet Explorer
|
|
7.0 mangles them beyond intelligibility. For serious internationalization purposes,
|
|
this is not an option.</p>
|
|
|
|
<p>The other possibility is to set Accept-Encoding to UTF-8, which
|
|
begs the question: Why aren't you using UTF-8 for everything then?
|
|
This route is more palatable, but there's a notable caveat: your data
|
|
will come in as UTF-8, so you will have to explicitly convert it into
|
|
your favored local character encoding.</p>
|
|
|
|
<p>I object to this approach on idealogical grounds: you're
|
|
digging yourself deeper into
|
|
the hole when you could have been converting to UTF-8
|
|
instead. And, of course, you can't use this method for GET requests.</p>
|
|
|
|
<h3 id="whyutf8-support">Well supported</h3>
|
|
|
|
<p>Almost every modern browser in the wild today has full UTF-8 and Unicode
|
|
support: the number of troublesome cases can be counted with the
|
|
fingers of one hand, and these browsers usually have trouble with
|
|
other character encodings too. Problems users usually encounter stem
|
|
from the lack of appropriate fonts to display the characters (once
|
|
again, this applies to all character encodings and HTML entities) or
|
|
Internet Explorer's lack of intelligent font picking (which can be
|
|
worked around).</p>
|
|
|
|
<p>We will go into more detail about how to deal with edge cases in
|
|
the browser world in the Migration section, but rest assured that
|
|
converting to UTF-8, if done correctly, will not result in users
|
|
hounding you about broken pages.</p>
|
|
|
|
<h3 id="whyutf8-htmlpurifier">HTML Purifier</h3>
|
|
|
|
<p>And finally, we get to HTML Purifier. HTML Purifier is built to
|
|
deal with UTF-8: any indications otherwise are the result of an
|
|
encoder that converts text from your preferred encoding to UTF-8, and
|
|
back again. HTML Purifier never touches anything else, and leaves
|
|
it up to the module iconv to do the dirty work.</p>
|
|
|
|
<p>This approach, however, is not perfect. iconv is blithely unaware
|
|
of HTML character entities. HTML Purifier, in order to
|
|
protect against sophisticated escaping schemes, normalizes all character
|
|
and numeric entities before processing the text. This leads to
|
|
one important ramification:</p>
|
|
|
|
<p><strong>Any character that is not supported by the target character
|
|
set, regardless of whether or not it is in the form of a character
|
|
entity or a raw character, will be silently ignored.</strong></p>
|
|
|
|
<p>Example of this principle at work: say you have <code>&theta;</code>
|
|
in your HTML, but the output is in Latin-1 (which, understandably,
|
|
does not understand Greek), the following process will occur (assuming you've
|
|
set the encoding correctly using %Core.Encoding):</p>
|
|
|
|
<ul>
|
|
<li>The <code>Encoder</code> will transform the text from ISO 8859-1 to UTF-8
|
|
(note that theta is preserved since it doesn't actually use
|
|
any non-ASCII characters): <code>&theta;</code></li>
|
|
<li>The <code>EntityParser</code> will transform all named and numeric
|
|
character entities to their corresponding raw UTF-8 equivalents:
|
|
<code>θ</code></li>
|
|
<li>HTML Purifier processes the code: <code>θ</code></li>
|
|
<li>The <code>Encoder</code> now transforms the text back from UTF-8
|
|
to ISO 8859-1. Since Greek is not supported by ISO 8859-1, it
|
|
will be either ignored or replaced with a question mark:
|
|
<code>?</code></li>
|
|
</ul>
|
|
|
|
<p>This behaviour is quite unsatisfactory. It is a deal-breaker for
|
|
international applications, and it can be mildly annoying for the provincial
|
|
soul who occasionally needs a special character. Since 1.4.0, HTML
|
|
Purifier has provided a slightly more palatable workaround using
|
|
%Core.EscapeNonASCIICharacters. The process now looks like:</p>
|
|
|
|
<ul>
|
|
<li>The <code>Encoder</code> transforms encoding to UTF-8: <code>&theta;</code></li>
|
|
<li>The <code>EntityParser</code> transforms entities: <code>θ</code></li>
|
|
<li>HTML Purifier processes the code: <code>θ</code></li>
|
|
<li>The <code>Encoder</code> replaces all non-ASCII characters
|
|
with numeric entities: <code>&#952;</code></li>
|
|
<li>For good measure, <code>Encoder</code> transforms encoding back to
|
|
original (which is strictly unnecessary for 99% of encodings
|
|
out there): <code>&#952;</code> (remember, it's all ASCII!)</li>
|
|
</ul>
|
|
|
|
<p>...which means that this is only good for an occasional foray into
|
|
the land of Unicode characters, and is totally unacceptable for Chinese
|
|
or Japanese texts. The even bigger kicker is that, supposing the
|
|
input encoding was actually ISO-8859-7, which <em>does</em> support
|
|
theta, the character would get entity-ized anyway! (The Encoder does
|
|
not discriminate).</p>
|
|
|
|
<p>The current functionality is about where HTML Purifier will be for
|
|
the rest of eternity. HTML Purifier could attempt to preserve the original
|
|
form of the entities so that they could be substituted back in, only the
|
|
DOM extension kills them off irreversibly. HTML Purifier could also attempt
|
|
to be smart and only convert non-ASCII characters that weren't supported
|
|
by the target encoding, but that would require reimplementing iconv
|
|
with HTML awareness, something I will not do.</p>
|
|
|
|
<p>So there: either it's UTF-8 or crippled international support. Your pick! (and I'm
|
|
not being sarcastic here: some people could care less about other languages)</p>
|
|
|
|
<h2 id="migrate">Migrate to UTF-8</h2>
|
|
|
|
<p>So, you've decided to bite the bullet, and want to migrate to UTF-8.
|
|
Note that this is not for the faint-hearted, and you should expect
|
|
the process to take longer than you think it will take.</p>
|
|
|
|
<p>The general idea is that you convert all existing text to UTF-8,
|
|
and then you set all the headers and META tags we discussed earlier
|
|
to UTF-8. There are many ways going about doing this: you could
|
|
write a conversion script that runs through the database and re-encodes
|
|
everything as UTF-8 or you could do the conversion on the fly when someone
|
|
reads the page. The details depend on your system, but I will cover
|
|
some of the more subtle points of migration that may trip you up.</p>
|
|
|
|
<h3 id="migrate-db">Configuring your database</h3>
|
|
|
|
<p>Most modern databases, the most prominent open-source ones being MySQL
|
|
4.1+ and PostgreSQL, support character encodings. If you're switching
|
|
to UTF-8, logically speaking, you'd want to make sure your database
|
|
knows about the change too. There are some caveats though:</p>
|
|
|
|
<h4 id="migrate-db-legit">Legit method</h4>
|
|
|
|
<p>Standardization in terms of SQL syntax for specifying character
|
|
encodings is notoriously spotty. Refer to your respective database's
|
|
documentation on how to do this properly.</p>
|
|
|
|
<p>For <a href="http://dev.mysql.com/doc/refman/5.0/en/charset-conversion.html">MySQL</a>, <code>ALTER</code> will magically perform the
|
|
character encoding conversion for you. However, you have
|
|
to make sure that the text inside the column is what is says it is:
|
|
if you had put Shift-JIS in an ISO 8859-1 column, MySQL will irreversibly mangle
|
|
the text when you try to convert it to UTF-8. You'll have to convert
|
|
it to a binary field, convert it to a Shift-JIS field (the real encoding),
|
|
and then finally to UTF-8. Many a website had pages irreversibly mangled
|
|
because they didn't realize that they'd been deluding themselves about
|
|
the character encoding all along, don't become the next victim.</p>
|
|
|
|
<p>For <a href="http://www.postgresql.org/docs/8.2/static/multibyte.html">PostgreSQL</a>, there appears to be no direct way to change the
|
|
encoding of a database (as of 8.2). You will have to dump the data, and then reimport
|
|
it into a new table. Make sure that your client encoding is set properly:
|
|
this is how PostgreSQL knows to perform an encoding conversion.</p>
|
|
|
|
<p>Many times, you will be also asked about the "collation" of
|
|
the new column. Collation is how a DBMS sorts text, like ordering
|
|
B, C and A into A, B and C (the problem gets surprisingly complicated
|
|
when you get to languages like Thai and Japanese). If in doubt,
|
|
going with the default setting is usually a safe bet.</p>
|
|
|
|
<p>Once the conversion is all said and done, you still have to remember
|
|
to set the client encoding (your encoding) properly on each database
|
|
connection using <code>SET NAMES</code> (which is standard SQL and is
|
|
usually supported).</p>
|
|
|
|
<h4 id="migrate-db-binary">Binary</h4>
|
|
|
|
<p>Due to the abovementioned compatibility issues, a more interoperable
|
|
way of storing UTF-8 text is to stuff it in a binary datatype.
|
|
<code>CHAR</code> becomes <code>BINARY</code>, <code>VARCHAR</code> becomes
|
|
<code>VARBINARY</code> and <code>TEXT</code> becomes <code>BLOB</code>.
|
|
Doing so can save you some huge headaches:</p>
|
|
|
|
<ul>
|
|
<li>The syntax for binary data types is very portable,</li>
|
|
<li>MySQL 4.0 has <em>no</em> support for character encodings, so
|
|
if you want to support it you <em>have</em> to use binary,</li>
|
|
<li>MySQL, as of 5.1, has no support for four byte UTF-8 characters,
|
|
which represent characters beyond the basic multilingual
|
|
plane, and</li>
|
|
<li>You will never have to worry about your DBMS being too smart
|
|
and attempting to convert your text when you don't want it to.</li>
|
|
</ul>
|
|
|
|
<p>MediaWiki, a very prominent international application, uses binary fields
|
|
for storing their data because of point three.</p>
|
|
|
|
<p>There are drawbacks, of course:</p>
|
|
|
|
<ul>
|
|
<li>Database tools like PHPMyAdmin won't be able to offer you inline
|
|
text editing, since it is declared as binary,</li>
|
|
<li>It's not semantically correct: it's really text not binary
|
|
(lying to the database),</li>
|
|
<li>Unless you use the not-very-portable wizardry mentioned above,
|
|
you have to change the encoding yourself (usually, you'd do
|
|
it on the fly), and</li>
|
|
<li>You will not have collation.</li>
|
|
</ul>
|
|
|
|
<p>Choose based on your circumstances.</p>
|
|
|
|
<h3 id="migrate-editor">Text editor</h3>
|
|
|
|
<p>For more flat-file oriented systems, you will often be tasked with
|
|
converting reams of existing text and HTML files into UTF-8, as well as
|
|
making sure that all new files uploaded are properly encoded. Once again,
|
|
I can only point vaguely in the right direction for converting your
|
|
existing files: make sure you backup, make sure you use
|
|
<a href="http://php.net/ref.iconv">iconv</a>(), and
|
|
make sure you know what the original character encoding of the files
|
|
is (or are, depending on the tidiness of your system).</p>
|
|
|
|
<p>However, I can proffer more specific advice on the subject of
|
|
text editors. Many text editors have notoriously spotty Unicode support.
|
|
To find out how your editor is doing, you can check out <a
|
|
href="http://www.alanwood.net/unicode/utilities_editors.html">this list</a>
|
|
or <a href="http://en.wikipedia.org/wiki/Comparison_of_text_editors#Encoding_support">Wikipedia's list.</a>
|
|
I personally use Notepad++, which works like a charm when it comes to UTF-8.
|
|
Usually, you will have to <strong>explicitly</strong> tell the editor through some dialogue
|
|
(usually Save as or Format) what encoding you want it to use. An editor
|
|
will often offer "Unicode" as a method of saving, which is
|
|
ambiguous. Make sure you know whether or not they really mean UTF-8
|
|
or UTF-16 (which is another flavor of Unicode).</p>
|
|
|
|
<p>The two things to look out for are whether or not the editor
|
|
supports <strong>font mixing</strong> (multiple
|
|
fonts in one document) and whether or not it adds a <strong>BOM</strong>.
|
|
Font mixing is important because fonts rarely have support for every
|
|
language known to mankind: in order to be flexible, an editor must
|
|
be able to take a little from here and a little from there, otherwise
|
|
all your Chinese characters will come as nice boxes. We'll discuss
|
|
BOM below.</p>
|
|
|
|
<h3 id="migrate-bom">Byte Order Mark (headers already sent!)</h3>
|
|
|
|
<p>The BOM, or <a href="http://en.wikipedia.org/wiki/Byte_Order_Mark">Byte
|
|
Order Mark</a>, is a magical, invisible character placed at
|
|
the beginning of UTF-8 files to tell people what the encoding is and
|
|
what the endianness of the text is. It is also unnecessary.</p>
|
|
|
|
<p>Because it's invisible, it often
|
|
catches people by surprise when it starts doing things it shouldn't
|
|
be doing. For example, this PHP file:</p>
|
|
|
|
<pre><strong>BOM</strong><?php
|
|
header('Location: index.php');
|
|
?></pre>
|
|
|
|
<p>...will fail with the all too familiar <strong>Headers already sent</strong>
|
|
PHP error. And because the BOM is invisible, this culprit will go unnoticed.
|
|
My suggestion is to only use ASCII in PHP pages, but if you must, make
|
|
sure the page is saved WITHOUT the BOM.</p>
|
|
|
|
<blockquote class="aside">
|
|
<p>The headers the error is referring to are <strong>HTTP headers</strong>,
|
|
which are sent to the browser before any HTML to tell it various
|
|
information. The moment any regular text (and yes, a BOM counts as
|
|
ordinary text) is output, the headers must be sent, and you are
|
|
not allowed to send anymore. Thus, the error.</p>
|
|
</blockquote>
|
|
|
|
<p>If you are reading in text files to insert into the middle of another
|
|
page, it is strongly advised (but not strictly necessary) that you replace out the UTF-8 byte
|
|
sequence for BOM <code>"\xEF\xBB\xBF"</code> before inserting it in,
|
|
via:</p>
|
|
|
|
<pre>$text = str_replace("\xEF\xBB\xBF", '', $text);</pre>
|
|
|
|
<h3 id="migrate-fonts">Fonts</h3>
|
|
|
|
<p>Generally speaking, people who are having trouble with fonts fall
|
|
into two categories:</p>
|
|
|
|
<ul>
|
|
<li>Those who want to
|
|
use an extremely obscure language for which there is very little
|
|
support even among native speakers of the language, and</li>
|
|
<li>Those where the primary language of the text is
|
|
well-supported but there are occasional characters
|
|
that aren't supported.</li>
|
|
</ul>
|
|
|
|
<p>Yes, there's always a chance where an English user happens across
|
|
a Sinhalese website and doesn't have the right font. But an English user
|
|
who happens not to have the right fonts probably has no business reading Sinhalese
|
|
anyway. So we'll deal with the other two edge cases.</p>
|
|
|
|
<h4 id="migrate-fonts-obscure">Obscure scripts</h4>
|
|
|
|
<p>If you run a Bengali website, you may get comments from users who
|
|
would like to read your website but get heaps of question marks or
|
|
other meaningless characters. Fixing this problem requires the
|
|
installation of a font or language pack which is often highly
|
|
dependent on what the language is. <a href="http://bn.wikipedia.org/wiki/%E0%A6%89%E0%A6%87%E0%A6%95%E0%A6%BF%E0%A6%AA%E0%A7%87%E0%A6%A1%E0%A6%BF%E0%A6%AF%E0%A6%BC%E0%A6%BE:Bangla_script_display_help">Here is an example</a>
|
|
of such a help file for the Bengali language, I am sure there are
|
|
others out there too. You just have to point users to the appropriate
|
|
help file.</p>
|
|
|
|
<h4 id="migrate-fonts-occasional">Occasional use</h4>
|
|
|
|
<p>A prime example of when you'll see some very obscure Unicode
|
|
characters embedded in what otherwise would be very bland ASCII are
|
|
letters of the
|
|
<a href="http://en.wikipedia.org/wiki/International_Phonetic_Alphabet">International
|
|
Phonetic Alphabet (IPA)</a>, use to designate pronounciations in a very standard
|
|
manner (you probably see them all the time in your dictionary). Your
|
|
average font probably won't have support for all of the IPA characters
|
|
like ʘ (bilabial click) or ʒ (voiced postalveolar fricative).
|
|
So what's a poor browser to do? Font mix! Smart browsers like Mozilla Firefox
|
|
and Internet Explorer 7 will borrow glyphs from other fonts in order
|
|
to make sure that all the characters display properly.</p>
|
|
|
|
<p>But what happens when the browser isn't smart and happens to be the
|
|
most widely used browser in the entire world? Microsoft IE 6
|
|
is not smart enough to borrow from other fonts when a character isn't
|
|
present, so more often than not you'll be slapped with a nice big �.
|
|
To get things to work, MSIE 6 needs a little nudge. You could configure it
|
|
to use a different font to render the text, but you can acheive the same
|
|
effect by selectively changing the font for blocks of special characters
|
|
to known good Unicode fonts.</p>
|
|
|
|
<p>Fortunantely, the folks over at Wikipedia have already done all the
|
|
heavy lifting for you. Get the CSS from the horses mouth here:
|
|
<a href="http://en.wikipedia.org/wiki/MediaWiki:Common.css">Common.css</a>,
|
|
and search for ".IPA" There are also a smattering of
|
|
other classes you can use for other purposes, check out
|
|
<a href="http://meta.wikimedia.org/wiki/Help:Special_characters#Displaying_Special_Characters">this page</a>
|
|
for more details. For you lazy ones, this should work:</p>
|
|
|
|
<pre>.Unicode {
|
|
font-family: Code2000, "TITUS Cyberbit Basic", "Doulos SIL",
|
|
"Chrysanthi Unicode", "Bitstream Cyberbit",
|
|
"Bitstream CyberBase", Thryomanes, Gentium, GentiumAlt,
|
|
"Lucida Grande", "Arial Unicode MS", "Microsoft Sans Serif",
|
|
"Lucida Sans Unicode";
|
|
font-family /**/:inherit; /* resets fonts for everyone but IE6 */
|
|
}</pre>
|
|
|
|
<p>The standard usage goes along the lines of <code><span class="Unicode">Crazy
|
|
Unicode stuff here</span></code>. Characters in the
|
|
<a href="http://en.wikipedia.org/wiki/Windows_Glyph_List_4">Windows Glyph List</a>
|
|
usually don't need to be fixed, but for anything else you probably
|
|
want to play it safe. Unless, of course, you don't care about IE6
|
|
users.</p>
|
|
|
|
<h3 id="migrate-variablewidth">Dealing with variable width in functions</h3>
|
|
|
|
<p>When people claim that PHP6 will solve all our Unicode problems, they're
|
|
misinformed. It will not fix any of the abovementioned troubles. It will,
|
|
however, fix the problem we are about to discuss: processing UTF-8 text
|
|
in PHP.</p>
|
|
|
|
<p>PHP (as of PHP5) is blithely unaware of the existence of UTF-8 (with a few
|
|
notable exceptions). Sometimes, this will cause problems, other times,
|
|
this won't. So far, we've avoided discussing the architecture of
|
|
UTF-8, so, we must first ask, what is UTF-8? Yes, it supports Unicode,
|
|
and yes, it is variable width. Other traits:</p>
|
|
|
|
<ul>
|
|
<li>Every character's byte sequence is unique and will never be found
|
|
inside the byte sequence of another character,</li>
|
|
<li>UTF-8 may use up to four bytes to encode a character,</li>
|
|
<li>UTF-8 text must be checked for well-formedness,</li>
|
|
<li>Pure ASCII is also valid UTF-8, and</li>
|
|
<li>Binary sorting will sort UTF-8 in the same order as Unicode.</li>
|
|
</ul>
|
|
|
|
<p>Each of these traits affect different domains of text processing
|
|
in different ways. It is beyond the scope of this document to explain
|
|
what precisely these implications are. PHPWact provides
|
|
a very good <a href="http://www.phpwact.org/php/i18n/utf-8">reference document</a>
|
|
on what to expect from each functions, although coverage is spotty in
|
|
some areas. Their more general notes on
|
|
<a href="http://www.phpwact.org/php/i18n/charsets">character sets</a>
|
|
are also worth looking at for information on UTF-8. Some rules of thumb
|
|
when dealing with Unicode text:</p>
|
|
|
|
<ul>
|
|
<li>Do not EVER use functions that:<ul>
|
|
<li>...convert case (strtolower, strtoupper, ucfirst, ucwords)</li>
|
|
<li>...claim to be case-insensitive (str_ireplace, stristr, strcasecmp)</li>
|
|
</ul></li>
|
|
<li>Think twice before using functions that:<ul>
|
|
<li>...count characters (strlen will return bytes, not characters;
|
|
str_split and word_wrap may corrupt)</li>
|
|
<li>...entity-ize things (UTF-8 doesn't need entities)</li>
|
|
<li>...do very complex string processing (*printf)</li>
|
|
</ul></li>
|
|
</ul>
|
|
|
|
<p>Note: this list applies to UTF-8 encoded text only: if you have
|
|
a string that you are 100% sure is ASCII, be my guest and use
|
|
<code>strtolower</code> (HTML Purifier uses this function.)</p>
|
|
|
|
<p>Regardless, always think in bytes, not characters. If you use strpos()
|
|
to find the position of a character, it will be in bytes, but this
|
|
usually won't matter since substr() also operates with byte indices!</p>
|
|
|
|
<p>You'll also need to make sure your UTF-8 is well-formed and will
|
|
probably need replacements for some of these functions. I recommend
|
|
using Harry Fuecks' <a href="http://phputf8.sourceforge.net/">PHP
|
|
UTF-8</a> library, rather than use mb_string directly. HTML Purifier
|
|
also defines a few useful UTF-8 compatible functions: check out
|
|
<code>Encoder.php</code> in the <code>/library/HTMLPurifier/</code>
|
|
directory.</p>
|
|
|
|
<h2 id="externallinks">Further Reading</h2>
|
|
|
|
<p>Well, that's it. Hopefully this document has served as a very
|
|
practical springboard into knowledge of how UTF-8 works. You may have
|
|
decided that you don't want to migrate yet: that's fine, just know
|
|
what will happen to your output and what bug reports you may recieve.</p>
|
|
|
|
<p>Many other developers have already discussed the subject of Unicode,
|
|
UTF-8 and internationalization, and I would like to defer to them for
|
|
a more in-depth look into character sets and encodings.</p>
|
|
|
|
<ul>
|
|
<li><a href="http://www.joelonsoftware.com/articles/Unicode.html">
|
|
The Absolute Minimum Every Software Developer Absolutely,
|
|
Positively Must Know About Unicode and Character Sets
|
|
(No Excuses!)</a> by Joel Spolsky, provides a <em>very</em>
|
|
good high-level look at Unicode and character sets in general.</li>
|
|
<li><a href="http://en.wikipedia.org/wiki/UTF-8">UTF-8 on Wikipedia</a>,
|
|
provides a lot of useful details into the innards of UTF-8, although
|
|
it may be a little off-putting to people who don't know much
|
|
about Unicode to begin with.</li>
|
|
</ul>
|
|
|
|
</body>
|
|
</html>
|