PHP OOP with Polymorphism

Shape.php

<?php 

// POLYMORPHISM

ini_set('display_startup_errors', 1);
ini_set('display_errors', 1);
error_reporting(-1);

interface Shape{
    public function calcArea();
}

class Circle implements Shape{

    private $r = 0;

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

    public function calcArea(){
        return 3.14 * $this->r * $this->r;
    }
}

class Rectangle implements Shape{

    private $x = 0;
    private $y = 0;

    public function __construct($x, $y){
        $this->x = $x;
        $this->y = $y;
    }

    public function calcArea(){
        return $this->x * $this->y;
    }
}

$c = new Circle(10);
$r = new Rectangle(7, 10);
$cArea = $c->calcArea();
$rArea = $r->calcArea();

echo '<pre>';
var_dump($cArea);
var_dump($rArea);
echo '</pre>';


?>