It seems like you call parent on a different method (key() instead of next()). Are you sure this is correct? If so, you might want to change this to $this->key().
This check looks for a call to a parent method whose name is different than
the method from which it is called.
This check marks calls to methods that do not seem to exist on an object.
This is most likely the result of a method being renamed without all references to it being renamed likewise.