How to implement the polymorphism principle?

In order to implement the polymorphism principle, we can choose between abstract classes and interfaces.

In order to ensure that the classes do implement the polymorphism principle, we can choose between one of the two options of either abstract classes or interfaces.

In the example given below, the interface with the name of Shape commits all the classes that implement it to define an abstract method with the name of calcArea().

interface Shape {
  public function calcArea();
}

In accordance, the Circle class implements the interface by putting into the calcArea() method the formula that calculates the area of circles.

class Circle implements Shape {
  private $radius;
   
  public function __construct($radius)
  {
    $this -> radius = $radius;
  }
  
  // calcArea calculates the area of circles 
  public function calcArea()
  {
    return $this -> radius * $this -> radius * pi();
  }
}

The rectangle class also implements the Shape interface but defines the method calcArea() with a calculation formula that is suitable for rectangles:

class Rectangle implements Shape {
  private $width;
  private $height;
   
  public function __construct($width, $height)
  {
    $this -> width = $width;
    $this -> height = $height;
  }
  
  // calcArea calculates the area of rectangles   
  public function calcArea()
  {
    return $this -> width * $this -> height;
  }
}

Now, we can create objects from the concrete classes:

$circ = new Circle(3);
$rect = new Rectangle(3,4);

We can be sure that all of the objects calculate the area with the method that has the name of calcArea(), whether it is a rectangle object or a circle object (or any other shape), as long as they implement the Shape interface.

Now, we can use the calcArea() methods to calculate the area of the shapes:

echo $circ -> calcArea();
echo $rect -> calcArea();

Result:
28.274333882308
12