Найди ошибки в коде

Необходимо найти ошибки в коде, предложить улучшения:

phpclass Shape
{
    public $width;
    public $height;

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

class Triangle
{
    public $radius;

    public function __construct($radius)
    {
        $this->radius = $radius;
    }
}
class AreaCalculator
{
    public
    function calculate($shapes)
    {
        foreach ($shapes as $shape) {
            if (is_a($shape, 'Square')) {
                $area = $shape->width * $shape->height;
            } else if (is_a($shape, 'Triangle')) {
                $area = $shape->radius * $shape->radius * pi();
            }
        }
        return array_sum($area);
    }
}