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<h1>Constants</h1>
<h2>Table of Contents</h2><ul class="chunklist chunklist_chapter"><li><a href="language.constants.syntax.html">Syntax</a></li><li><a href="language.constants.predefined.html">Magic constants</a></li></ul>
<p class="simpara">
A constant is an identifier (name) for a simple value. As the name
suggests, that value cannot change during the execution of the
script (except for <a href="language.constants.predefined.html" class="link">
magic constants</a>, which aren't actually constants).
A constant is case-sensitive by default. By convention, constant
identifiers are always uppercase.
</p>
<p class="para">
The name of a constant follows the same rules as any label in PHP. A
valid constant name starts with a letter or underscore, followed
by any number of letters, numbers, or underscores. As a regular
expression, it would be expressed thusly:
<em>[a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*</em>
</p>
<div class="tip"><strong class="tip">Tip</strong><p class="simpara">See also the
<a href="userlandnaming.html" class="xref">Userland Naming Guide</a>.</p></div>
<p class="para">
<div class="example" id="example-112">
<p><strong>Example #1 Valid and invalid constant names</strong></p>
<div class="example-contents">
<div class="phpcode"><code><span style="color: #000000">
<span style="color: #0000BB"><?php<br /><br /></span><span style="color: #FF8000">// Valid constant names<br /></span><span style="color: #0000BB">define</span><span style="color: #007700">(</span><span style="color: #DD0000">"FOO"</span><span style="color: #007700">, </span><span style="color: #DD0000">"something"</span><span style="color: #007700">);<br /></span><span style="color: #0000BB">define</span><span style="color: #007700">(</span><span style="color: #DD0000">"FOO2"</span><span style="color: #007700">, </span><span style="color: #DD0000">"something else"</span><span style="color: #007700">);<br /></span><span style="color: #0000BB">define</span><span style="color: #007700">(</span><span style="color: #DD0000">"FOO_BAR"</span><span style="color: #007700">, </span><span style="color: #DD0000">"something more"</span><span style="color: #007700">);<br /><br /></span><span style="color: #FF8000">// Invalid constant names<br /></span><span style="color: #0000BB">define</span><span style="color: #007700">(</span><span style="color: #DD0000">"2FOO"</span><span style="color: #007700">, </span><span style="color: #DD0000">"something"</span><span style="color: #007700">);<br /><br /></span><span style="color: #FF8000">// This is valid, but should be avoided:<br />// PHP may one day provide a magical constant<br />// that will break your script<br /></span><span style="color: #0000BB">define</span><span style="color: #007700">(</span><span style="color: #DD0000">"__FOO__"</span><span style="color: #007700">, </span><span style="color: #DD0000">"something"</span><span style="color: #007700">); <br /><br /></span><span style="color: #0000BB">?></span>
</span>
</code></div>
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</p>
<blockquote class="note"><p><strong class="note">Note</strong>:
<span class="simpara">
For our purposes here, a letter is a-z, A-Z, and the ASCII
characters from 127 through 255 (0x7f-0xff).
</span>
</p></blockquote>
<p class="simpara">
Like <a href="language.variables.predefined.html" class="link">superglobals</a>, the scope of a constant is global. You
can access constants anywhere in your script without regard to scope.
For more information on scope, read the manual section on
<a href="language.variables.scope.html" class="link">variable scope</a>.
</p>
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