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htmlpurifier/docs/enduser-utf8.html
Edward Z. Yang 0ead9558b4 Finish up to BOM. 
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<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en"><head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
<meta name="description" content="Describes the rationale for using UTF-8, the ramifications otherwise, and how to make the switch." />
<link rel="stylesheet" type="text/css" href="./style.css" />
<script defer="defer" type="text/javascript" src="./toc-gen.js"></script>
<style type="text/css">
.minor td {font-style:italic;}
</style>
<title>UTF-8 - HTML Purifier</title>
<!-- Note to users: this document, though professing to be UTF-8, attempts
to use only ASCII characters, because most webservers are configured
to send HTML as ISO-8859-1. So I will, many times, go against my
own advice for sake of portability. -->
</head><body>
<h1>UTF-8</h1>
<div id="filing">Filed under End-User</div>
<div id="index">Return to the <a href="index.html">index</a>.</div>
<div id="home"><a href="http://hp.jpsband.org/">HTML Purifier</a> End-User Documentation</div>
<p>Character encoding and character sets, in truth, are not that
difficult to understand. But if you don't understand them, you are going
to be caught by surprise by some of HTML Purifier's behavior, namely
the fact that it operates UTF-8 or the limitations of the character
encoding transformations it does. This document will walk you through
determining the encoding of your system and how you should handle
this information. It will stay away from excessive discussion on
the internals of character encoding, but offer the information in
asides that can easily be skipped.</p>
<blockquote class="aside">
<div class="label">Asides</div>
<p>Text in this formatting is an <strong>aside</strong>,
interesting tidbits for the curious but not strictly necessary material to
do the tutorial. If you read this text, you'll come out
with a greater understanding of the underlying issues.</p>
</blockquote>
<h2 id="findcharset">Finding the real encoding</h2>
<p>In the beginning, there was ASCII, and things were simple. But they
weren't good, for no one could write in Cryllic or Thai. So there
exploded a proliferation of character encodings to remedy the problem
by extending the characters ASCII could express. This ridiculously
simplified version of the history of character encodings shows us that
there are now many character encodings floating around.</p>
<blockquote class="aside">
<p>A <strong>character encoding</strong> tells the computer how to
interpret raw zeroes and ones into real characters. It
usually does this by pairing numbers with characters.</p>
<p>There are many different types of character encodings floating
around, but the ones we deal most frequently with are ASCII,
8-bit encodings, and Unicode-based encodings.</p>
<ul>
<li><strong>ASCII</strong> is a 7-bit encoding based on the
English alphabet.</li>
<li><strong>8-bit encodings</strong> are extensions to ASCII
that add a potpourri of useful, non-standard characters
like &eacute; and &aelig;. They can only add 127 characters,
so usually only support one script at a time. When you
see a page on the web, chances are it's encoded in one
of these encodings.</li>
<li><strong>Unicode-based encodings</strong> implement the
Unicode standard and include UTF-8, UCS-2 and UTF-16.
They go beyond 8-bits (the first two are variable length,
while the second one uses 16-bits), and support almost
every language in the world. UTF-8 is gaining traction
as the dominant international encoding of the web.</li>
</ul>
</blockquote>
<p>The first step of our journey is to find out what the encoding of
your website is. The most reliable way is to ask your
browser:</p>
<dl>
<dt>Mozilla Firefox</dt>
<dd>Tools &gt; Page Info: Encoding</dd>
<dt>Internet Explorer</dt>
<dd>View &gt; Encoding: bulleted item is unofficial name</dd>
</dl>
<p>Internet Explorer won't give you the mime (i.e. useful/real) name of the
character encoding, so you'll have to look it up using their description.
Some common ones:</p>
<table class="table">
<thead><tr>
<th>IE's Description</th>
<th>Mime Name</th>
</tr></thead>
<tbody>
<tr><th colspan="2">Windows</th></tr>
<tr><td>Arabic (Windows)</td><td>Windows-1256</td></tr>
<tr><td>Baltic (Windows)</td><td>Windows-1257</td></tr>
<tr><td>Central European (Windows)</td><td>Windows-1250</td></tr>
<tr><td>Cyrillic (Windows)</td><td>Windows-1251</td></tr>
<tr><td>Greek (Windows)</td><td>Windows-1253</td></tr>
<tr><td>Hebrew (Windows)</td><td>Windows-1255</td></tr>
<tr><td>Thai (Windows)</td><td>TIS-620</td></tr>
<tr><td>Turkish (Windows)</td><td>Windows-1254</td></tr>
<tr><td>Vietnamese (Windows)</td><td>Windows-1258</td></tr>
<tr><td>Western European (Windows)</td><td>Windows-1252</td></tr>
</tbody>
<tbody>
<tr><th colspan="2">ISO</th></tr>
<tr><td>Arabic (ISO)</td><td>ISO-8859-6</td></tr>
<tr><td>Baltic (ISO)</td><td>ISO-8859-4</td></tr>
<tr><td>Central European (ISO)</td><td>ISO-8859-2</td></tr>
<tr><td>Cyrillic (ISO)</td><td>ISO-8859-5</td></tr>
<tr class="minor"><td>Estonian (ISO)</td><td>ISO-8859-13</td></tr>
<tr class="minor"><td>Greek (ISO)</td><td>ISO-8859-7</td></tr>
<tr><td>Hebrew (ISO-Logical)</td><td>ISO-8859-8-l</td></tr>
<tr><td>Hebrew (ISO-Visual)</td><td>ISO-8859-8</td></tr>
<tr class="minor"><td>Latin 9 (ISO)</td><td>ISO-8859-15</td></tr>
<tr class="minor"><td>Turkish (ISO)</td><td>ISO-8859-9</td></tr>
<tr><td>Western European (ISO)</td><td>ISO-8859-1</td></tr>
</tbody>
<tbody>
<tr><th colspan="2">Other</th></tr>
<tr><td>Chinese Simplified (GB18030)</td><td>GB18030</td></tr>
<tr><td>Chinese Simplified (GB2312)</td><td>GB2312</td></tr>
<tr><td>Chinese Simplified (HZ)</td><td>HZ</td></tr>
<tr><td>Chinese Traditional (Big5)</td><td>Big5</td></tr>
<tr><td>Japanese (Shift-JIS)</td><td>Shift_JIS</td></tr>
<tr><td>Japanese (EUC)</td><td>EUC-JP</td></tr>
<tr><td>Korean</td><td>EUC-KR</td></tr>
<tr><td>Unicode (UTF-8)</td><td>UTF-8</td></tr>
</tbody>
</table>
<p>Internet Explorer does not recognize some of the more obscure
character encodings, and having to lookup the real names with a table
is a pain, so I recommend using Mozilla Firefox to find out your
character encoding.</p>
<h2 id="findmetacharset">Finding the embedded encoding</h2>
<p>At this point, you may be asking, &quot;Didn't we already find out our
encoding?&quot; Well, as it turns out, there are multiple places where
a web developer can specify a character encoding, and one such place
is in a <code>META</code> tag:</p>
<pre>&lt;meta http-equiv=&quot;Content-Type&quot; content=&quot;text/html; charset=UTF-8&quot; /&gt;</pre>
<p>You'll find this in the <code>HEAD</code> section of an HTML document.
The text to the right of <code>charset=</code> is the &quot;claimed&quot;
encoding: the HTML claims to be this encoding, but whether or not this
is actually the case depends on other factors. For now, take note
if your <code>META</code> tag claims that either:</p>
<ol>
<li>The character encoding is the same as the one reported by the
browser,</li>
<li>The character encoding is different from the browser's, or</li>
<li>There is no <code>META</code> tag at all! (horror, horror!)</li>
</ol>
<h2 id="fixcharset">Fixing the encoding</h2>
<p>If your <code>META</code> encoding and your real encoding match,
savvy! You can skip this section. If they don't...</p>
<h3 id="fixcharset-none">No embedded encoding</h3>
<p>If this is the case, you'll want to add in the appropriate
<code>META</code> tag to your website. It's as simple as copy-pasting
the code snippet above and replacing UTF-8 with whatever is the mime name
of your real encoding.</p>
<blockquote class="aside">
<p>For all those skeptics out there, there is a very good reason
why the character encoding should be explicitly stated. When the
browser isn't told what the character encoding of a text is, it
has to guess: and sometimes the guess is wrong. Hackers can manipulate
this guess in order to slip XSS pass filters and then fool the
browser into executing it as active code. A great example of this
is the <a href="http://shiflett.org/archive/177">Google UTF-7
exploit</a>.</p>
<p>You might be able to get away with not specifying a character
encoding with the <code>META</code> tag as long as your webserver
sends the right Content-Type header, but why risk it? Besides, if
the user downloads the HTML file, there is no longer any webserver
to define the character encoding.</p>
</blockquote>
<h3 id="fixcharset-diff">Embedded encoding disagrees</h3>
<p>This is an extremely common mistake: another source is telling
the browser what the
character encoding is and is overriding the embedded encoding. This
source usually is the Content-Type HTTP header that the webserver (i.e.
Apache) sends. A usual Content-Type header sent with a page might
look like this:</p>
<pre>Content-Type: text/html; charset=ISO-8859-1</pre>
<p>Notice how there is a charset parameter: this is the webserver's
way of telling a browser what the character encoding is, much like
the <code>META</code> tags we touched upon previously.</p>
<blockquote class="aside"><p>In fact, the <code>META</code> tag is
designed as a substitute for the HTTP header for contexts where
sending headers is impossible (such as locally stored files without
a webserver). Thus the name <code>http-equiv</code> (HTTP equivalent).
</p></blockquote>
<p>There are two ways to go about fixing this: changing the <code>META</code>
tag to match the HTTP header, or changing the HTTP header to match
the <code>META</code> tag. How do we know which to do? It depends
on the website's content: after all, headers and tags are only ways of
describing the actual characters on the web page.</p>
<p>If your website:</p>
<dl>
<dt>...only uses ASCII characters,</dt>
<dd>Either way is fine, but I recommend switching both to
UTF-8 (more on this later).</dd>
<dt>...uses special characters, and they display
properly,</dt>
<dd>Change the embedded encoding to the server encoding.</dd>
<dt>...uses special characters, but users often complain that
they come out garbled,</dt>
<dd>Change the server encoding to the embedded encoding.</dd>
</dl>
<p>Changing a META tag is easy: just swap out the old encoding
for the new. Changing the server (HTTP header) encoding, however,
is slightly more difficult.</p>
<h3 id="fixcharset-server">Changing the server encoding</h3>
<h4 id="fixcharset-server-php">PHP header() function</h4>
<p>The simplest way to handle this problem is to send the encoding
yourself, via your programming language. Since you're using HTML
Purifier, I'll assume PHP, although it's not too difficult to do
similar things in
<a href="http://www.w3.org/International/O-HTTP-charset#scripting">other
languages</a>. The appropriate code is:</p>
<pre><a href="http://php.net/function.header">header</a>('Content-Type:text/html; charset=UTF-8');</pre>
<p>...replacing UTF-8 with whatever your embedded encoding is.
This code must come before any output, so be careful about
stray whitespace in your application.</p>
<h4 id="fixcharset-server-phpini">PHP ini directive</h4>
<p>PHP also has a neat little ini directive that can save you a
header call: <code><a href="http://php.net/ini.core#ini.default-charset">default_charset</a></code>. Using this code:</p>
<pre><a href="http://php.net/function.ini_set">ini_set</a>('default_charset', 'UTF-8');</pre>
<p>...will also do the trick. If PHP is running as an Apache module (and
not as FastCGI, consult
<a href="http://php.net/phpinfo">phpinfo</a>() for details), you can even use htaccess do apply this property
globally:</p>
<pre><a href="http://php.net/configuration.changes#configuration.changes.apache">php_value</a> default_charset &quot;UTF-8&quot;</pre>
<blockquote class="aside"><p>As with all INI directives, this can
also go in your php.ini file. Some hosting providers allow you to customize
your own php.ini file, ask your support for details. Use:</p>
<pre>default_charset = &quot;utf-8&quot;</pre></blockquote>
<h4 id="fixcharset-server-nophp">Non-PHP</h4>
<p>You may, for whatever reason, may need to set the character encoding
on non-PHP files, usually plain ol' HTML files. Doing this
is more of a hit-or-miss process: depending on the software being
used as a webserver and the configuration of that software, certain
techniques may work, or may not work.</p>
<h4 id="fixcharset-server-htaccess">.htaccess</h4>
<p>On Apache, you can use an .htaccess file to change the character
encoding. I'll defer to
<a href="http://www.w3.org/International/questions/qa-htaccess-charset">W3C</a>
for the in-depth explanation, but it boils down to creating a file
named .htaccess with the contents:</p>
<pre><a href="http://httpd.apache.org/docs/1.3/mod/mod_mime.html#addcharset">AddCharset</a> UTF-8 .html</pre>
<p>Where UTF-8 is replaced with the character encoding you want to
use and .html is a file extension that this will be applied to. This
character encoding will then be set for any file directly in
or in the subdirectories of directory you place this file in.</p>
<p>If you're feeling particularly courageous, you can use:</p>
<pre><a href="http://httpd.apache.org/docs/1.3/mod/core.html#adddefaultcharset">AddDefaultCharset</a> UTF-8</pre>
<p>...which changes the character set Apache adds to any document that
doesn't have any Content-Type parameters. This directive, which the
default configuration file sets to iso-8859-1 for security
reasons, is probably why your headers mismatch
with the <code>META</code> tag. If you would prefer Apache not to be
butting in on your character encodings, you can tell it not
to send anything at all:</p>
<pre><a href="http://httpd.apache.org/docs/1.3/mod/core.html#adddefaultcharset">AddDefaultCharset</a> Off</pre>
<p>...making your <code>META</code> tags the sole source of
character encoding information. In these cases, it is
<em>especially</em> important to make sure you have valid <code>META</code>
tags on your pages and all the text before them is ASCII.</p>
<blockquote class="aside"><p>These directives can also be
placed in httpd.conf file for Apache, but
in most shared hosting situations you won't be able to edit this file.
</p></blockquote>
<h4 id="fixcharset-server-ext">File extensions</h4>
<p>If you're not allowed to use .htaccess files, you can often
piggy-back off of Apache's default AddCharset declarations to get
your files in the proper extension. Here are Apache's default
character set declarations:</p>
<table class="table">
<thead><tr>
<th>Charset</th>
<th>File extension(s)</th>
</tr></thead>
<tbody>
<tr><td>ISO-8859-1</td><td>.iso8859-1 .latin1</td></tr>
<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>&lt;?xml version=&quot;1.0&quot; encoding=&quot;UTF-8&quot;?&gt;</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, be sure to make sure it jives
with your <code>META</code> tags and HTTP headers.</p>
<h3>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 &quot;Why are there
so many options?&quot; 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, &theta; can be
written <code>&amp;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: &#28608;&#20809;,
&#36889;&#20841;&#20491;&#23383;&#26159;&#29978;&#40636;&#24847;&#24605;.
The entity-ized version would look like this:</p>
<pre>&amp;#28608;&amp;#20809;, &amp;#36889;&amp;#20841;&amp;#20491;&amp;#23383;&amp;#26159;&amp;#29978;&amp;#40636;&amp;#24847;&amp;#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, &quot;intelligible&quot; character
entities like <code>&amp;theta;</code> will leave users who are
uninterested in learning HTML scratching their heads. On the other
hand, if they see &theta; 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
&quot;smart&quot; 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 I18N 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>&amp;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>&amp;theta;</code></li>
<li>The <code>EntityParser</code> will transform all named and numeric
character entities to their corresponding raw UTF-8 equivalents:
<code>&theta;</code></li>
<li>HTML Purifier processes the code: <code>&theta;</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
I18N 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>&amp;theta;</code></li>
<li>The <code>EntityParser</code> transforms entities: <code>&theta;</code></li>
<li>HTML Purifier processes the code: <code>&theta;</code></li>
<li>The <code>Encoder</code> replaces all non-ASCII characters
with numeric entities: <code>&amp;#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>&amp;#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 I18N 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 &quot;collation&quot; 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 I18N 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.
You will usually 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 &quot;Unicode&quot; 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>&lt;?php
header('Location: index.php');
?&gt;</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 that you replace out the UTF-8 byte
sequence for BOM <code>&quot;\xEF\xBB\xBF&quot;</code> before inserting it in.</p>
<h3 id="migrate-fonts">Fonts</h3>
<h3 id="migrate-variablewidth">Dealing with variable width in functions</h3>
<h2 id="externallinks">Further Reading</h2>
<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>
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