In a new post to his blog Brandon Savage writes about a method he's found to make testing Zend Framework applications simpler - specifically in implementing the right interfaces to get the job done.

The unfortunate thing is that more PHP developers don't implement interfaces when they could or should. They prefer to implement abstract or concrete classes and extend from there; however, interfaces offer three distinct advantages over inheritance in [the case of testing a Zend_Auth part of your application].

He gives a few reasons why interfaces should be used including the fact that more than one can be implemented at once, that they make mocking objects easy and that, since it defines most of the API up front, you can know that those methods will be there in the inheriting classes.