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<?xml version="1.0" encoding="utf-8"?>
<!-- $Revision: 330913 $ -->
<sect1 xml:id="language.oop5.constants" xmlns="http://docbook.org/ns/docbook">
<title>Class Constants</title>
<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 <varname>$</varname> symbol to declare or use them.
</para>
<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.
</para>
<para>
It's also possible for interfaces to have <literal>constants</literal>. Look at
the <link linkend="language.oop5.interfaces">interface documentation</link> for
examples.
</para>
<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. <literal>self</literal>,
<literal>parent</literal> and <literal>static</literal>).
</para>
<example>
<title>Defining and using a constant</title>
<programlisting role="php">
<![CDATA[
<?php
class MyClass
{
const CONSTANT = 'constant value';
function showConstant() {
echo self::CONSTANT . "\n";
}
}
echo MyClass::CONSTANT . "\n";
$classname = "MyClass";
echo $classname::CONSTANT . "\n"; // As of PHP 5.3.0
$class = new MyClass();
$class->showConstant();
echo $class::CONSTANT."\n"; // As of PHP 5.3.0
?>
]]>
</programlisting>
</example>
<example>
<title>Static data example</title>
<programlisting role="php">
<![CDATA[
<?php
class foo {
// As of PHP 5.3.0
const BAR = <<<'EOT'
bar
EOT;
}
?>
]]>
</programlisting>
<para>
Unlike heredocs, nowdocs can be used in any static data context.
</para>
</example>
<note>
<para>
Nowdoc support was added in PHP 5.3.0.
</para>
</note>
</sect1>
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