Specifications

Declaring B does not affect the original definition of A. Consider the following two lines of

code:

$a = new A();

$a -> operation();

We have created an object of type A and called its operation() function. This will produce

Something

The value of $attribute is default value

proving that creating B has not altered A. If we create an object of type B, we will get different

output.

This code

$b = new B();

$b -> operation();

will produce

Something else

The value of $attribute is different value

In the same way that providing new attributes or operations in a subclass does not affect the

superclass, overriding attributes or operations in a subclass does not affect the superclass.

A subclass will inherit all the attributes and operations of its superclass, unless you provide

replacements. If you provide a replacement definition, this takes precedence and overrides the

original definition.

Unlike some other OO languages, PHP does not allow you to override a function and still be

able to call the version defined in the parent.

Inheritance can be many layers deep. We can declare a class imaginatively called C, that

extends B and therefore inherits features from B and from B’s parent A. The class C can again

choose which attributes and operations from its parents to override and replace.

Multiple Inheritance

Some OO languages support multiple inheritance, but PHP does not. This means that each

class can only inherit from one parent. No restrictions exist for how many children can share a

single parent.

It might not seem immediately clear what this means. Figure 6.1 shows three different ways

that three classes named A, B, and C can inherit.

Object-Oriented PHP

HAPTER 6

OBJECT-ORIENTED

PHP

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