OOP Abstract Classes and Methods

An abstract class provides a partial implementation that other classes can build upon. In addition to normal class members, it may contain incomplete methods that must be implemented in child classes.

A class designed only as a parent from which sub-classes may be derived.

  • Abstract classes are classes that may contain abstract method(s).
  • An abstract method is a method that is declared but contains no implementation.
  • If a class contains an abstract method, the class itself must be declared as abstract.
  • Abstract classes can not be instantiated, and require subclasses to provide implementations for the abstract methods.

To make a class abstract, add the keyword abstract before the class name in the class definition.

 abstract class Products {
 protected $name;
 protected $type;

 public function __construct($name, $type){
  $this->type = $type;
  $this->name = $name;

 public function getType() {
  return $this->type;

 public function getName() {
  return $this->name;

//In an abstract class, any method can be declared abstract
 abstract public function getPrice();

Abstract classes can not be instantiated, so we’ll extend it by making a child class Paper. An abstract method getPrice declared in the abstract class. A method marked abstract in the parent’s class declaration must be defined by the inherited (child) class.

 class Paper extends Products {
  public function getPrice() {
   $a = [
   'A4' =>     ['80g' => 8.49, '100g' => 10.49],
   'Letter' => ['80g' => 8.99, '100g' => 10.99]
   return $a[$this -> name][$this -> type];

$product = new Paper('A4','80g');
echo $product->getPrice(); # Prints: 8.49

$product = new Paper('Letter','100g');
echo $product->getPrice(); # Prints: 10.99

So a class that extends an abstract class must implement all abstract methods. The implementing method should also require the same number of arguments as the abstract method or you’ll receive a fatal error message.

For example, the abstract method getPrice in our abstract class Products does not have any argument, if you declare the argument when implementing the method, a fatal error will occur:

 class Paper extends Products {
  public function getPrice($a) {
   // ...

# Fatal error: Declaration of Paper::getPrice($a) must be compatible with Products::getPrice()

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