class Point
{
    function __construct($x = 0, $y = 0, $z = 0)
    {
        $this->x = $x;
        $this->y = $y;
        $this->z = $z;
    }
    
    public function show() {
        echo "($this->x, $this->y, $this->z)";
    }
}

class Point2D extends Point {
    
    function __construct($x = 0, $y = 0) {
        parent::__construct($x, $y, 0);
    }
}

$p = new Point2D(10, 20);
$p->show();