Specifications

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 life. You 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 character-

istics 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 subclasses. A

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 derive more complex and specialized classes as the need arises. This makes your code

more reusable, which is one of the important advantages of an object-oriented approach.

Using inheritance might save us work if operations can be written once in a superclass rather

than many times in separate subclasses. It might also allow us to more accurately model real-

world relationships. If a sentence about two classes makes sense with “is a” between the

classes, inheritance is probably appropriate. The sentence “a car is a vehicle” makes sense, but

the sentence “a vehicle is a car” does not make sense because not all vehicles are cars.

Therefore, car can inherit from vehicle.

Creating Classes, Attributes, Operations in PHP

So far, we have discussed classes in a fairly abstract way. When creating a class in PHP, you

must use the keyword class.

Using PHP

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