OOP: Abstraction (abstract classes)

<?php

abstract class Shape{
     abstract  function draw(); /// no need to have a definition

     public function render(){ /// this uses the draw method from other classes..
            return "Render : " . $this->draw();
     }
}

class Circle extends Shape{

    public function draw(){
        return "The circle is drawn";
    }
}

class Rectangle extends Shape{
    public function draw(){
        return "The rectangle is drawn";
    }
}


$circleTest = new Circle();
//echo $circleTest->render();

$rectangleTest = new Rectangle();
//echo $rectangleTest->render();