public static function addProduct($id)
 {
     if (!isset($_SESSION["cart"])) {
         $_SESSION["cart"] = [];
     }
     $product = ProductManager::getProduct($id);
     $cartProduct = new CartProduct($product, 1);
     if (array_key_exists($cartProduct->getId(), $_SESSION["cart"])) {
         $quantity = $_SESSION["cart"][$cartProduct->getId()]->getQuantity();
         $_SESSION["cart"][$cartProduct->getId()]->setQuantity($quantity + 1);
     } else {
         $_SESSION["cart"][$cartProduct->getId()] = $cartProduct;
     }
 }
 public static function update(CartProduct $oCartProduct)
 {
     if (!array_key_exists('cart', $_SESSION)) {
         $_SESSION['cart'] = array();
     }
     $_SESSION['cart'][$oCartProduct->getId()] = $oCartProduct->getQuantity();
 }