Advanced OOP Features - Class Inheritance

As applied to PHP, class inheritance is accomplished by using the extends keyword. Listing 7-3 demonstrates this ability, first creating anEmployeeclass, and then creating anExecutive class that inherits fromEmployee.

¦Note  A class that inherits from another class is known as a child class, or a subclass. The class from which the child class inherits is known as the parent, or base class.

Listing 7-3. Inheriting from a Base Class

<?php
   # Define a base Employee class
   class Employee {

      private $name;

      # Define a setter for the private $name member.
      function setName($name) {
         if ($name == "") echo "Name cannot be blank!";
         else $this->name = $name;
      }

      # Define a getter for the private $name member
      function getName() {
         return "My name is ".$this->name."<br />";
      }
   } #end Employee class

   # Define an Executive class that inherits from Employee
   class Executive extends Employee {
      # Define a method unique to Employee
      function pillageCompany() {
         echo "I'm selling company assets to finance my yacht!";
      }
   } #end Executive class

   # Create a new Executive object
   $exec = new Executive();

   # Call the setName() method, defined in the Employee class
   $exec->setName("Richard");

   # Call the getName() method
   echo $exec->getName();

   # Call the pillageCompany() method
   $exec->pillageCompany();
?>

This returns the following:

--------------------------------------------
My name is Richard.
I'm selling company assets to finance my yacht!
--------------------------------------------

Because all employees have a name, theExecutiveclass inherits from theEmployeeclass, saving you the hassle of having to re-create thename member and the corresponding getter and setter. You can then focus solely on those characteristics that are specific to an executive, in this case a method namedpillageCompany(). This method is available solely to objects of typeExecutive, and not to theEmployeeclass or any other class, unless of course we create a class that inherits fromExecutive. The following example demonstrates that concept, producing a class titledCEO, which inherits fromExecutive:

<?php 

   class Employee {
   ...
   }

   class Executive extends Employee {
   ...
   }

   class CEO extends Executive {
      function getFacelift() {
         echo "nip nip tuck tuck";
      }
   }

   $ceo = new CEO();
   $ceo->setName("Bernie");
   $ceo->pillageCompany();
   $ceo->getFacelift();

?>

BecauseExecutivehas inherited fromEmployee, objects of typeCEOalso have all the members and methods that are available toExecutive.

Please check back next week for the second part of this series.