the getorderByCart function simply returns the order_id when the input is a cart_id, located in /classes/order.php the function is as follows
/**
* Get an order by its cart id
*
* @param integer $id_cart Cart id
* @return array Order details
*/
static public function getOrderByCartId($id_cart)
{
$result = Db::getInstance()->getRow('
SELECT `id_order`
FROM `'._DB_PREFIX_.'orders`
WHERE `id_cart` = '.intval($id_cart));
return isset($result['id_order']) ? $result['id_order'] : false;
}example of use
$myid_order = Order::getOrderByCartId(intval($id_cart));
