This appears to be a bug not a feature request, as it should be possible. e.g. in the code below the userland interface + abstract class is fine. But the internal interface and abstract class is not fine.

<?php 

interface foo {
    public function bar();
}


abstract class b implements foo {} 
//compiles fine

abstract class a implements Traversable {}
//PHP Fatal error:  Class a must implement interface Traversable as part of either Iterator or IteratorAggregate in Unknown on line 0