Problema OOP la zencart

am nevoie sa adaug un produs intr-o clasa de sesiune, la afisare print_r($_SESSION['cart']) imi apare urmatorul sir:

shoppingCart Object ( [contents] => Array ( [3152] => Array ( [qty] => 4 ) [3256] => Array ( [qty] => 1 ) ) [total] => 12.5 [weight] => 0 [cartID] => 00739 [content_type] => [free_shipping_item] => 0 [free_shipping_weight] => 0 [free_shipping_price] => 0 [observers] => Array ( ) )

$_SESSION['cart']->add_cart($products_id, $qty = '1', $attributes = '');

Multumesc mult, te-as mai ruga sa-mi zici cum pot update la cantitate daca produsul exista

$_SESSION['cart']->update_quantity($products_id, $quantity, $attributes);

Daca te uiti prin clasa "shoppingCart"(/includes/classes/shopping_cart.php) o sa te prinzi cum sta treaba daca mai ai nelamuriri.