Polymorphism with PHP

What is Polymorphism?

The general definition is this: The ability of an object to exist in various forms.
In particular with programming the term is this: Polymorphism describes a pattern in object oriented programming in which classes have different functionality while sharing a common interface.

<?php

interface Shape
{
    public function calcArea();
}

class Circle implements Shape
{
    private $radius;

    public function __construct($radius)
    {
        $this->radius = $radius;
    }

    public function calcArea()
    {
        return $this->radius * $this->radius * pi();
    }
}

class Rectangle implements Shape
{
    private $width;
    private $height;

    public function __construct($width, $height)
    {
        $this->width  = $width;
        $this->height = $height;
    }

    public function calcArea()
    {
        return $this->width * $this->height;
    }
}

$circ = new Circle(2);
$rect = new Rectangle(3,4);

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

// 12.566370614359
// 12

And this is it. The Circle and the Rectangle share the same interface, but their formulas for calculating the object areas are totally different.