Abstract methods and abstract classes for exploring object models in PHP5.0

The object-oriented program is built through the layered structure of the class. In a single repeated language such as PHP, the class continues to be tree-like. A root class has one or more sub-classes, and then one or more sub-classes continue from each sub-class. Of course, there may be multiple root classes to implement different functions. In a well-designed system, each root class should have a useful interface that can be used by application code. If our application code is designed to work with the root class, it can also work with any subclass that continues from the root class.

An abstract method is a placeholder of a method as in a subclass (which occupies a place but does not work). It is different from a general method.