defined

(PHP 3, PHP 4 )

defined --  Checks whether a given named constant exists

Description

bool defined ( string name)

Returns TRUE if the named constant given by name has been defined, FALSE otherwise.

Пример 1. Checking Constants

<?php
if (defined("CONSTANT")){ // Note that it should be quoted
    echo CONSTANT;
    }
?>

See also define(), constant(), get_defined_constants() and the section on Constants.