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

170 Chapter 6 Object-Oriented PHP
class B extends A
{
function __construct()
{
$this->operation1();
$this->operation2();
$this->operation3();
}
}
$b = new B;
?>
This code creates one operation of each type in class A: public, protected, and
private. B inherits from A. In the constructor of B, you then try to call the operations
from the parent.
The line
$this->operation1();
produces a fatal error as follows:
Fatal error: Call to private method A::operation1() from context ‘B’
This example shows that private operations cannot be called from a child class.
If you comment out this line, the other two function calls will work.The protected
function is inherited but can be used only from inside the child class, as done here. If
you try adding the line
$b->operation2();
to the bottom of the file, you will get the following error:
Fatal error: Call to protected method A::operation2() from context ‘’
However, you can call operation3() from outside the class, as follows:
$b->operation3();
You can make this call because it is declared as public.
Overriding
In this chapter, we have shown a subclass declaring new attributes and operations. It is
also valid and sometimes useful to redeclare the same attributes and operations.You
might do this to give an attribute in the subclass a different default value to the same
attribute in its superclass or to give an operation in the subclass different functionality to
the same operation in its superclass.This action is called overriding.