Classes and Objects (PHP 5)
PHP Manual

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#1 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";
?>

Classes and Objects (PHP 5)
PHP Manual