defined Checks whether a constant with the given name exists &reftitle.description; booldefined stringconstant_name Checks whether a constant with the given constant_name is defined. This function works also with class constants and enum cases. 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; constant_name The constant name. &reftitle.returnvalues; Returns &true; if the named constant given by constant_name has been defined, &false; otherwise. &reftitle.examples; ]]> ]]> &reftitle.seealso; define constant get_defined_constants function_exists The section on Constants