PHP中不再强制必需的参数？

Example:

public function a($a, $b){

}


a();  // warning: missing arguments. why no fatal error???

If I try to force a fatal error

public function a($a, $b){

  if(!$a || !$b)  // <- notice undefined variable. wtf?
    throw new exception('!!!');
}

a();

I'm pretty sure this wasn't happening before :|