defined Checks whether a given named constant exists &reftitle.description; booldefined stringname Checks whether the given constant exists and is defined. 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. &reftitle.parameters; name The constant name. &reftitle.returnvalues; Returns &true; if the named constant given by name has been defined, &false; otherwise. &reftitle.examples; ]]> &reftitle.seealso; define constant get_defined_constants function_exists The section on Constants