On PHPMaster.com there's a new tutorial from Alejandro Gervasio about subtype polymorphism. It sounds a little scary, but really it's just a look at sub-objects that inherit from parents and how to swap them around at runtime to do different things.
In this article I’ll show you how to exploit the virtues that Polymorphism offers through the development of a pluggable cache component. The core functionality can be expanded later to suit your needs through the development of additional cache drivers.
He starts off with the definition of an interface to provide structure to the sample application (the CacheInterface) and implements it in two different subtypes - a FileCache and an AppCache (using the file system and APC, respectively). He builds on these and creates a "View" that uses these caching systems to generate and save the output to a cache for use later in the execution.