Polymorphism Using Interface in PHP OOP

<?php

interface Shape
{
    public function area();

    public function perimeter();
}

class Circle implements Shape
{
    private $radius;

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

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

    public function perimeter()
    {
        return 2  * $this->radius * pi();
    }
}

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

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

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

    public function perimeter()
    {
        return ($this->width + $this->height) * 2;
    }
}

$circle1 = new Circle(5);
$circle2 = new Circle(9);
$rectangle1 = new Rectangle(5, 20);
$rectangle2 = new Rectangle(7, 28);
$shapes = [$circle1, $circle2, $rectangle1, $rectangle2];
foreach ($shapes as $shape) {
    echo '<br>area: ' . $shape->area();
    echo '<br>perimeter: ' . $shape->perimeter();
    echo '<br>----------------------------------';
}

?>

Output

area: 78.539816339745
perimeter: 31.415926535898
----------------------------------
area: 254.46900494077
perimeter: 56.548667764616
----------------------------------
area: 100
perimeter: 50
----------------------------------
area: 196
perimeter: 70
----------------------------------