function_exists

Description

Checks the list of defined functions, both built-in (internal) and user-defined, for function_name. Returns TRUE on success or FALSE on failure.

Note that a function name may exist even if the function itself is unusable due to configuration or compiling options (with the image functions being an example). Also note that function_exists() will return FALSE for constructs, such as include_once() and echo().

See also method_exists() and get_defined_functions().