PHP OOP - Abstraction

In this tutorial we will learn about abstraction in PHP.

What is Abstraction?

In OOP, abstraction is a concept in which a class has methods without implementation. The idea is to have a template and let the child class that inherits the parent class implement the method.

How to create an abstract method?

We use the keyword abstract before the method name when we want to create an abstract method. If a class has an abstract method then we also add the abstract keyword before the class.

In the following example we have an abstract method foo() and so we have added abstract keyword to the class.

abstract class Sample {
	
	//abstract method
	public abstract function foo();

}

When we create an abstract class we are actually creating a template.

We can't instantiate an abstract class

Since an abstract class is just a template without completely implemented methods hence it is not allowed to create its object.

//abstract class
abstract class Sample {
	
}

//this will give error - can't create object of an abstract class
$obj = new Sample();

An abstract class can have fully implemented methods

It is allowed to have fully implemented methods inside an abstract class. In the following example we have an abstract class Sample with an abstract method foo() and fully implemented method hello().

//abstract class
abstract class Sample {
	
	//abstract method
	public abstract function foo();

	//method
	public function hello() {
		printf("Hello World!");
	}

}

Inherit an abstract class

When an abstract class is inherited by the child class, the abstract method of the parent class must be implemented in the child class.

If the child class is not implementing an abstract method of the parent class then the child class must be declared abstract.

Child class implementing abstract method

In the following example we have an abstract parent class Sample having an abstract method foo() and add(). We have a child class Po which extends the parent class Sample and implements the abstract method foo() and add().

//abstract parent class
abstract class Sample {
	
	//abstract method
	public abstract function foo();

	public abstract function add($x, $y);

}

//child class
class Po extends Sample {
	
	//implementing abstract methods of parent class

	public function foo() {
		printf("Hello World!");
	}

	public function add($x, $y) {
		return ($x + $y);
	}

}

//create object of the Po class
$obj = new Po();

$obj->foo();	//this will print "Hello World!"

$sum = $obj->add(10, 20);	//this will return 30

printf("Sum = %d", $sum);	//this will print "Sum = 30"

Child class not implementing abstract method

It is not mandatory for the child class to implement an abstract method of a parent class.

If a child class is not implementing an abstract method of a parent class then the child class must be declared abstract.

In the following example we have a parent class Sample having abstract method foo() and hello(). We have a child class Po that extends the parent class and only implements the abstract method hello() and does not implement the abstract method foo().

Since the child class Po has one unimplemented abstract method foo() hence we have to declare the class Po as abstract class.

//abstract parent class
abstract class Sample {
	
	//abstract method
	public abstract function foo();

	public abstract function hello();
}

/**
 * child class Po set to abstract
 * as it is not implementing the abstract method foo()
 */
abstract class Po extends Sample {
	
	//implemented method
	public function hello() {
		printf("Hello World!");
	}

}