. PHP Manual . Prev . Next . (PHP 3 <= 3.0.18, PHP 4 >= 4.0.0) defined -- Checks whether a given named constant exists . bool defined ( string name) . Returns TRUE if the named constant given by name has been defined, FALSE otherwise. Example 1. Checking Constants . <?php if (defined("CONSTANT")){ // Note that it should be quoted echo CONSTANT; // } ?> . See also define() , constant() and the section on Constants . Prev . Home . Next . define . Up . die