Personally I believe that you should always use require (require_once) instead of include (include_once) to be sure that a failure to load file creates an E_ERROR instead of an E_WARNING, this both for loading the framework but also when the framework when loading the api file.

E_ERROR is a fatal error thus it will throw (I think since PHP 7) an Error exception that can be catched with a normal try/catch and thus possible to return a proper response.