Source of: /manual/en/language.oop5.constants.php
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<div id="language.oop5.constants" class="sect1">
<h2 class="title">Class Constants</h2>
<p class="para">
It is possible to define constant values on a per-class basis remaining the
same and unchangeable. Constants differ from normal variables in that you
don't use the <var class="varname">$</var> symbol to declare or use them.
</p>
<p class="para">
The value must be a constant expression, not (for example) a variable, a
property, a result of a mathematical operation, or a function call.
</p>
<p class="para">
Its also possible for interfaces to have <i>constants</i>. Look at
the <a href="language.oop5.interfaces.php" class="link">interface documentation</a> for
examples.
</p>
<p class="para">
As of PHP 5.3.0, it's possible to reference the class using a variable.
The variable's value can not be a keyword (e.g. <i>self</i>,
<i>parent</i> and <i>static</i>).
</p>
<div class="example">
<p><b>Example #1 Defining and using a constant</b></p>
<div class="example-contents programlisting">
<div class="phpcode"><code><span style="color: #000000">
<span style="color: #0000BB"><?php<br /></span><span style="color: #007700">class </span><span style="color: #0000BB">MyClass<br /></span><span style="color: #007700">{<br /> const </span><span style="color: #0000BB">constant </span><span style="color: #007700">= </span><span style="color: #DD0000">'constant value'</span><span style="color: #007700">;<br /><br /> function </span><span style="color: #0000BB">showConstant</span><span style="color: #007700">() {<br /> echo </span><span style="color: #0000BB">self</span><span style="color: #007700">::</span><span style="color: #0000BB">constant </span><span style="color: #007700">. </span><span style="color: #DD0000">"\n"</span><span style="color: #007700">;<br /> }<br />}<br /><br />echo </span><span style="color: #0000BB">MyClass</span><span style="color: #007700">::</span><span style="color: #0000BB">constant </span><span style="color: #007700">. </span><span style="color: #DD0000">"\n"</span><span style="color: #007700">;<br /><br /></span><span style="color: #0000BB">$classname </span><span style="color: #007700">= </span><span style="color: #DD0000">"MyClass"</span><span style="color: #007700">;<br />echo </span><span style="color: #0000BB">$classname</span><span style="color: #007700">::</span><span style="color: #0000BB">constant </span><span style="color: #007700">. </span><span style="color: #DD0000">"\n"</span><span style="color: #007700">; </span><span style="color: #FF8000">// As of PHP 5.3.0<br /><br /></span><span style="color: #0000BB">$class </span><span style="color: #007700">= new </span><span style="color: #0000BB">MyClass</span><span style="color: #007700">();<br /></span><span style="color: #0000BB">$class</span><span style="color: #007700">-></span><span style="color: #0000BB">showConstant</span><span style="color: #007700">();<br /><br />echo </span><span style="color: #0000BB">$class</span><span style="color: #007700">::</span><span style="color: #0000BB">constant</span><span style="color: #007700">.</span><span style="color: #DD0000">"\n"</span><span style="color: #007700">; </span><span style="color: #FF8000">// As of PHP 5.3.0<br /></span><span style="color: #0000BB">?></span>
</span>
</code></div>
</div>
</div>
<div class="example">
<p><b>Example #2 Static data example</b></p>
<div class="example-contents programlisting">
<div class="phpcode"><code><span style="color: #000000">
<span style="color: #0000BB"><?php<br /></span><span style="color: #007700">class </span><span style="color: #0000BB">foo </span><span style="color: #007700">{<br /> </span><span style="color: #FF8000">// As of PHP 5.3.0<br /> </span><span style="color: #007700">const </span><span style="color: #0000BB">bar </span><span style="color: #007700">= <<<'EOT'<br /></span><span style="color: #DD0000">bar<br /></span><span style="color: #007700">EOT;<br />}<br /></span><span style="color: #0000BB">?></span>
</span>
</code></div>
</div>
<div class="example-contents para"><p>
Unlike heredocs, nowdocs can be used in any static data context.
</p></div>
</div>
<blockquote><p><b class="note">Note</b>:
Nowdoc support was added in PHP 5.3.0.
<br />
</p></blockquote>
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