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 default visibility of class constants is public.

The value must be a constant expression, not (for example) a variable, a property, or a function call.

It’s also possible for interfaces to have constants. Look at the interface documentation for examples.

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. self, parent and static).

Note that class constants are allocated once per class, and not for each class instance.

• Class constants are public
• Class constants are being inherited
• Class constants can be initialized by const

<?php
 class MyClass {
 const CONSTANT = 'constant value';

function showConstant() {
 echo self::CONSTANT . "\n";
 }
 }
 
 echo MyClass::CONSTANT . "\n";
 
 $class = new MyClass();
 $class->showConstant();
 
?>