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Finish up to BOM.

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Edward Z. Yang 2007-01-24 01:29:25 +00:00
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@ -676,18 +676,162 @@ not being sarcastic here: some people could care less about other languages)</p>
<h2 id="migrate">Migrate to UTF-8</h2>
<h3 id="migrate-editor">Text editor</h3>
<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>
<h3 id="migrate-convert">Convert old text</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>
<h3 id="migrate-variablewidth">Dealing with variable width in functions</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,