Some concepts of object-oriented design are harder to figure out that others for a budding programmer. Thankfully, there's articles like this one from DevShed to help fill in some of the gaps. Specifically, they focus on inheritance, polymorphism, and serialization in your PHP classes.
If you are working with classes in PHP, you will sooner or later encounter inheritance, polymorphism, and serialization. The ability to use these three will help speed up your code writing. This article covers how to use them, and more.
First off, they look at inheritance and working with child classes before moving on to polymorphism (similar functionality, different objects), calling functions statically, and serializing objects to make passing them around much easier.