PHP Define

<?php
define("CONSTANT", "value");

echo CONSTANT;
?>

<?php
define("CONSTANT", "Hello world.");
echo CONSTANT."<br>"; // outputs "Hello world."
echo Constant."<br>"; // outputs "Constant" and issues a notice.

define("GREETING", "Hello you.", true);
echo GREETING."<br>"; // outputs "Hello you."
echo Greeting."<br>"; // outputs "Hello you."
?>