Syntax
You can define a constant by using the define()-function or by using the const keyword outside a class definition as of PHP 5.3.0. Once a constant is defined, it can never be changed or undefined.
Note: Constants and (global) variables are in a different namespace. This implies that for example
TRUEand $TRUE are generally different.
Example #1 Defining Constants
<?php
define("CONSTANT", "Hello world.");
echo CONSTANT; // outputs "Hello world."
echo Constant; // outputs "Constant" and issues a notice.
?>
Example #2 Defining Constants using the const keyword
<?php
// Works as of PHP 5.3.0
const CONSTANT = 'Hello World';
echo CONSTANT;
?>
Future Technology 