jigoshop_cart.class.php

    function add_to_cart($product_id, $quantity = 1, $variation_id = '', $variation = array(), $customer_design) {

        if ($quantity < 0) {
            $quantity = 0;
        }

        //customer design not present
        if (empty($customer_design)) {
            return false;
        }

        // Load cart item data - may be added by other plugins
        $cart_item_data = (array) apply_filters('jigoshop_add_cart_item_data', array(), $product_id);

        $cart_id = self::generate_cart_id($product_id, $variation_id, $variation, $cart_item_data);
        $cart_item_key = self::find_product_in_cart( $cart_id );

        if (empty($variation_id)) {
            $product = new jigoshop_product($product_id);
        } else {
            $product = new jigoshop_product_variation($variation_id);
        }

        //product with a given ID doesn't exists
        if (empty($product)) {
            return false;
        }

        // prevents adding products with no price to the cart
        if ($product->get_price() === '') {
            jigoshop::add_error(__('You cannot add this product to your cart because its price is not yet announced', 'jigoshop'));
            return false;
        }

        // prevents adding products to the cart without enough quantity on hand
        $in_cart_qty = is_numeric($cart_item_key) ? self::$cart_contents[$cart_item_key]['quantity'] : 0;
        if ($product->managing_stock() && !$product->has_enough_stock($quantity + $in_cart_qty)) :
            if ($in_cart_qty > 0) :
                $error = (self::get_options()->get_option('jigoshop_show_stock') == 'yes') ? sprintf(__('We are sorry.  We do not have enough "%s" to fill your request.  You have %d of them in your Cart and we have %d available at this time.', 'jigoshop'), $product->get_title(), $in_cart_qty, $product->get_stock()) : sprintf(__('We are sorry.  We do not have enough "%s" to fill your request.', 'jigoshop'), $product->get_title());
            else :
                $error = (self::get_options()->get_option('jigoshop_show_stock') == 'yes') ? sprintf(__('We are sorry.  We do not have enough "%s" to fill your request. There are only %d left in stock.', 'jigoshop'), $product->get_title(), $product->get_stock()) : sprintf(__('We are sorry.  We do not have enough "%s" to fill your request.', 'jigoshop'), $product->get_title());
            endif;
            jigoshop::add_error($error);
            return false;
        endif;

        //if product is already in the cart change its quantity
        if ($cart_item_key) {

            $quantity = (int) $quantity + self::$cart_contents[$cart_item_key]['quantity'];

            self::set_quantity($cart_item_key, $quantity);

        } else {
            // otherwise add new item to the cart
            self::$cart_contents[$cart_id] = apply_filters( 'jigoshop_add_cart_item', array(
                'data'        => $product,
                'product_id'  => $product_id,
                'quantity'    => (int) $quantity,
                'variation'   => $variation,
                'variation_id'=> $variation_id,
                'customer_design' => $customer_design
            ), $cart_item_data);
        }

        self::set_session();

        return true;
    }