defined

Description

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

Example 1. Checking Constants <?php /* Note the use of quotes, this is important.  This example is checking * if the string 'CONSTANT' is the name of a constant named CONSTANT */ if (defined('CONSTANT')) {     echo CONSTANT; } ?>

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