Circle.php

<?php

class Circle extends Shape {

    const SHAPE_TYPE = 3;
    protected $radius;

/*
Constructor accepts a radius parameter and intializes $radius.
*/
    function __construct($radius) {
        parent::__construct($length = null, $width = null);
        $this->radius = $radius;
    }

/*
area method to calculate and return the area of the circle (PI x r x r)
*/
    public function area() {
        $pi = pi();
        $area = $pi * pow($this->radius, 2);
        return $area;
    }

/*
getFullDescription method to return string describing the shape
Uses getId() to get the Id because that's a protected property of the parent class
*/
    public function getFullDescription() {
        return get_class($this) . '<#' . $this->getId() . '>: ' . $this->name . ' - ' . $this->radius;
    }
}