PHP simple shopping cart class design_PHP tutorial

class Product
{
	protected $_partNumber, $_description, $_price;
   
	public function __construct($parNumber,$description,$price)
	{
   	  	$this->_partNumber=$parNumber;
   	  	$this->_description=$description;
   		$this->_price=$price;
   	}
   
   	public function getPartNumber()
    {
   		return $this->_partNumber;
   	}
   
   	public function getDescription()
    {
   		return $this->_description;
   	}
   
   	public function getPrice()
    {
   		return $this->_price;
   	}
}

require_once ('Product.php');
class Cart extends ArrayObject
{
	protected $_products;
	
	public function __construct()
    {
		$this->_products=array();
		parent::__construct($this->_products);
	}
	
	public function getCarTotal()
    {
		for(
		  	$i=$sum=0,$cnt=count($this);
		  	$i<$cnt;
		  	$sum+=$this[$i++]->getPrice()
		);
		return $sum;
	}
}

$cart=new Cart();
$cart[]=new Product('00231-A','Description',1.99);
$cart[]=new Product('00231-B','B',1.99);
echo $cart->getCarTotal();