On the KillerPHP blog today, there's this new post that asks the question of when the choice should be made to use interfaces and why they should be used instead of classes in your code. It's an article/podcast, so for the full effect, grab the audio here.
The post itself are really just notes to support the content in the podcast, but they provide plenty of information by themselves. They talk about what interfaces are, include examples of both them and a class (showing how they differ) and an example of a class using and interface (dog implementing animal).
There are also a few other notes there at the bottom mentioning the purpose of interfaces, which one to choose when, and two miscellaneous notes about how interfaces can help both you and your code.