Web Database Applications with PHP and MySQL, 2nd Edition

Chapter 14. Advanced Features of Object-Oriented Programming in PHP 5

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

Working with Class Hierarchies

In Chapter 4, we showed you how inheritance can be used as a powerful tool in object-oriented programming. Using the extends keyword, a class can inherit the capabilities from a parent class. As we explained in Chapter 4, PHP allows an inheritance from only one parent class, but a class can be extended to create any number of child classes.

Inheritance allows functionality to be built up from related classes. Each class provides specialized support that's specific to a class's purpose. It's common that functionality in an object-oriented application is provided by class hierarchies rather than single, unrelated classes.

Example 14-1 shows how a class hierarchy is formed to provide functionality that describes shapes.

Example 14-1. Shape classes

<?php class ...