Descriptionbool function_exists ( string function_name)
Checks the list of defined functions, both built-in (internal) and user-defined, for function_name. 如果成功则返回 TRUE，失败则返回 FALSE。
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().