Polymorphism in PHP

By: Andi, Stig and Derick  

The subject of polymorphism is probably the most important in OOP. Using classes and inheritance makes it easy to describe a real-life situation as opposed to just a collection of functions and data. They also make it much easier to grow projects by reusing code mainly via inheritance. Also, to write robust and extensible code, you usually want to have as few as possible flowcontrol statements (such as if() statements). Polymorphism answers all these needs and more.

Consider the following code:

class Cat {

function miau()

{

print "miau";

}

}

class Dog {

function wuff()

{

print "wuff";

}

}

function printTheRightSound($obj)

{

if ($obj instanceof Cat) {

$obj->miau();

} else if ($obj instanceof Dog) {

$obj->wuff();

} else {

print "Error: Passed wrong kind of object";

}

print "\n";

}

printTheRightSound(new Cat());

printTheRightSound(new Dog());

The output is

miau

wuff

You can easily see that this example is not extensible. Say that you want to extend it by adding the sounds of three more animals. You would have to add another three else if blocks to printTheRightSound() so you check that the object you have is an instance of one of those new animals, and then you have to add the code to call each sound method.

Polymorphism using inheritance solves this problem. It enables you to inherit from a parent class, inheriting all its methods and properties and thus creating an is-a relationship.

Taking the previous example, we will create a new class called Animal from which all other animal kinds will inherit, thus creating is-a relationships from the specific kinds, such as Dog, to the parent (or ancestor) Animal.

Inheritance is performed by using the extends keyword:

class Child extends Parent {

...

}

This is how you would rewrite the previous example using inheritance:

class Animal {

function makeSound()

{

print "Error: This method should be re-implemented in the children";

}

}

class Cat extends Animal {

function makeSound()

{

print "miau";

}

}

class Dog extends Animal {

function makeSound()

{

print "wuff";

}

}

function printTheRightSound($obj)

{

if ($obj instanceof Animal) {

$obj->makeSound();

} else {

print "Error: Passed wrong kind of object";

}

print "\n";

}

printTheRightSound(new Cat());

printTheRightSound(new Dog());

The output is

miau

wuff

You can see that no matter how many animal types you add to this example, you will not have to make any changes to printTheRightSound() because the instanceof Animal check covers all of them, and the $obj->makeSound() call will do so, too.

This example can still be improved upon. Certain modifiers available to you in PHP can give you more control over the inheritance process. For example, the class Animal and its method makeSound() can be marked as being abstract, which not only means that you don’t have to give some meaningless implementation for the makeSound() definition in the Animal class, but also forcing any inheriting classes to implement it. Additionally, we could specify access modifiers to the makeSound() method, such as the public modifier, meaning that it can be called anywhere in your code.

Note: PHP does not support multiple inheritance like C++ does. It supplies a different solution for creating more than one is-a relationship for a given class by using Java-like interfaces.




Archived Comments


Most Viewed Articles (in PHP )

Installing PHP with nginx-server under windows

PHP ./configure RESULTING IN [email protected]_2_2_3_... AND UNRESOLVED REFERENCES WITH ORACLE OCI8

PHP 5.1.4 INSTALLATION on Solaris 9 (Sparc)

Building PHP 5.x with Apache2 on SuSE Professional 9.1/9.2

Installing PHP 5.x with Apache 2.x on HP UX 11i and configuring PHP 5.x with Oracle 9i

Cannot load /usr/local/apache/libexec/libphp4.so into server: ld.so.1:......

Setting up PHP in Windows 2003 Server IIS7, and WinXP 64

error: "Service Unavailable" after installing PHP to a Windows XP x64 Pro

Running different websites on different versions of PHP in Windows 2003 & IIS6 platform

Function to convert strings to strict booleans in PHP

Convert IP address to integer and back to IP address in PHP

Function to return number of digits of an integer in PHP

Function to force strict boolean values in PHP

Function to sort array by elements and count of element in PHP

PHP pages does not display in IIS 6 with Windows 2003

Latest Articles (in PHP)

Comment on this tutorial