Well, if you are into duck typing style of programming, it may be interesting for you to have an object that implements certain set of functions, but not necessary declares it at class definition. Languages like Smalltalk do it all day along, so why PHP couldn't?

His example defines an interface Cow and a class MooingGrassEater and a function, CowConsumer, that does the work. A classname is passed in and an instance of that class is checked with "implements" rather than "instanceof" to see if it uses the Cow interface. He points out a place where PHP itself uses something similar in user defined streams.

For me the coolest feature of PersistentObject is, that the component does not require you're ORM enabled classes to inherit from a certain base to allow your objects to be stored in a database (made persistent).

This new object gives you a "wapper" of sorts to make any of the pre-existing objects in your application persistent. Check out the article for more.