If you want to check for included interface and you already register spl autoloader - it will crash. Becassue autoloader trying to load `string` and he doesnt matter is it class or not.
Iv found several ways :
1 - unregister AL - -> check for Ifaces - -> register Autoloader
2 - $ifaces = array_flip(get_declared_interfaces());
if($ifaces["MyIface"]) // empty // isset .
Interfaces are not bad, you can build correct geomentry of system , with validation by funcs / vars / const .
Also they are good to storage variables <?php
ROOT::THEMES ; ROOT::LOC ; ?> . Much faster then Define, but you cant put algorithms inside, only complite strings / __file__ / etc.