Call to undefined function doIt()

class Ball{
    const SHAPE = "circular";
    public $area = null;
}

class Football extends Ball{
    const PI = 3.14;
    public $radius;

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

    public function doIt(){
        $this->area = PI*pow($this->radius,2);
        return "the shape of the ball is:".self::SHAPE."the area is:".$this->area;
    }
}

$fb = new Football(2.15);
echo $fb.doIt();

詳細(xì)代碼如上，代碼編寫沒有問題，但是就是執(zhí)行不了，提示Call to undefined function doIt()，不知道怎么回事。