PHP and MySQL Web Development 4th Ed-tqw-_darksiderg

Implementing Inheritance in PHP
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Understanding Multiple Inheritance
A few OO languages (most notably C++ and Smalltalk) support multiple inheritance,
but like most, PHP does not.This means that each class can inherit from only one parent.
No restrictions exist for how many children can share a single parent.What this
means might not seem immediately clear. Figure 6.1 shows three different ways that
three classes named A, B, and C can inherit.
A
A
A
B
B
C
Single Inheritance
B
C
Single Inheritance
C
Multiple Inheritance
Figure 6.1
PHP does not support multiple inheritance.
The left combination shows class C inheriting from class B, which in turn inherits from
class A. Each class has at most one parent, so this is a perfectly valid single inheritance
in PHP.
The center combination shows classes B and C inheriting from class A. Each class has
at most one parent, so again this is a valid single inheritance.
The right combination shows class C inheriting from both class A and class B. In this
case, class C has two parents, so this is a case of multiple inheritance and is invalid in
PHP.
Implementing Interfaces
If you need to implement the functionality seen in instances of multiple inheritance, you
can do so in PHP through interfaces.They are seen as workarounds for multiple inheritance
and are similar to the interface implementation supported by other object-oriented
languages, including Java.
The idea of an interface is that it specifies a set of functions that must be implemented
in classes that implement that interface. For instance, you might decide that you have
a set of classes that need to be able to display themselves. Instead of having a parent class
with a display() function that they all inherit from and override, you can implement
an interface as follows:
interface Displayable
{