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