php-doc-en/language/oop5/constants.xml

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 <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
   class member, result of a mathematical operation or a function call.
  </para>

  <para>
   As of PHP 5.3.0, it's possible to reference the class using a variable.
   Keywords like <literal>self</literal>, <literal>parent</literal> or 
   <literal>static</literal> are not allowed in dynamic class references.
  </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";

$class = new MyClass();
$class->showConstant();

echo $class::constant."\n";
?>
]]>
   </programlisting>
  </example>

 </sect1>
 
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