(PHP 3, PHP 4, PHP 5)

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

/* 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')) {

Note: If you want to see if a variable exists, use isset() as defined() only applies to constants. If you want to see if a function exists, use function_exists().

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