On the XpertDeveloper.com site today there's a new tutorial talking about something that can not only help the structure of your application but can make things more reusable in the end - abstract classes.

For Abstact keyword we can say that, abstract is type of the class and class which we can't create a object of it. Surprised???? [...] Abstract class can be used some what like an interface in PHP. So basically we can implement class using abstract. We can't extend more than one abstract class while we can implement more than one interface.

They introduce you to the creation of an abstract class and show how to set up some abstract methods inside. These methods are required to be defined as a part of the extension in your class. One of the benefits they don't mention of abstract classes over interfaces is the ability to have methods in the abstract that are actual code, not just definitions of the structure (that's more of what interfaces are for).