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Class Constants

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 $ symbol to declare or use them.

The value must be a constant expression, not (for example) a variable, a class member, result of a mathematical operation or a function call.

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

Example 10.17. Defining and using a constant

<?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";
?>