tailieunhanh - PHP and MySQL Web Development - P37

PHP and MySQL Web Development - P37: PHP and MySQL Web Development teaches the reader to develop dynamic, secure, commercial Web sites. Using the same accessible, popular teaching style of the first edition, this best-selling book has been updated to reflect the rapidly changing landscape of MySQL and PHP. | Creating Classes Attributes Operations in PHP 147 Polymorphism An object-oriented programming language must support polymorphism which means that different classes can have different behaviors for the same operation. If for instance we have a class car and a class bicycle both can have different move operations. For real-world objects this would rarely be a problem. Bicycles are not likely to get confused and start using a car s move operation instead. However a programming language does not possess the common sense of the real world so the language must support polymorphism in order to know which move operation to use on a particular object. Polymorphism is more a characteristic of behaviors than it is of objects. In PHP only member functions of a class can be polymorphic. A real world comparison is that of verbs in natural languages which are equivalent to member functions. Consider the ways a bicycle can be used in real can clean it move it disassemble it repair it or paint it among other things. These verbs describe generic actions because you don t know what kind of object is being acted on. This type of abstraction of objects and actions is one of the distinguishing characteristics of human intelligence. For example moving a bicycle requires completely different actions from those required for moving a car even though the concepts are similar. The verb move can be associated with a particular set of actions only once the object acted on is made known. Inheritance Inheritance allows us to create a hierarchical relationship between classes using subclass inherits attributes and operations from its superclass. For example car and bicycle have some things in common. We could use a class vehicle to contain the things such as a color attribute and a move operation that all vehicles have and then let our car and bicycle classes inherit from vehicle. With inheritance you can build on and add to existing classes. From a simple base class you can