Saving an array to a mySQL table

I am writing a shopping cart. I am now ready to take the order from being in a session variable $_SESSION['product_selected'][$product_reference_number] to store it into the orders table. While the products selected are being displayed in a loop I have the piece of code $final_order .= $_SESSION['selection'][$product_reference] Of course this is only capturing the quantity of $product_reference, not assigning the array to $final_order and the quantity being ordered. I am trying to maintain inventory control in my shopping cart and want to be able to query the orders table to find out what products have left so I don't sell something I don't have. Ron