In Chapter 4, we introduced the basic concepts of object-oriented programming, showing how classes are defined and how objects are created and used with PHP. The techniques shown in Chapter 4 can be used to help build reusable, modular code, and are applicable when using object-oriented packages such as the PEAR packages we describe in Chapter 7. However, much of the power of object-oriented programming doesn't come from simply reusing code.
In this chapter, we show some more powerful techniques available with object-oriented programming, including:
Class hierarchies
Polymorphic behavior
Class type hints
Abstract classes and interfaces