Variabel post problem

Begriber det ikke, det hele fungerer fint, dog poster den samme værdi til alle poster i amount..


for ($i=0, $n=sizeof($order->products); $i<$n; $i++) {

        $product          = $order->products[$i]['id'];   
        $product_tjek      = $_POST[$product];
       
        $product_qty      = $order->products[$i]['id'] . '_qty';
        $product_tjek_qty = $_POST[$product_qty];
       
        if ($product_tjek != ''){

        $resultat = mysql_query("SELECT * FROM balance_of_order WHERE product = $product LIMIT 0,1");
       
        if (mysql_num_rows($resultat) >= 1){
        mysql_query("UPDATE balance_of_order SET amount = '$product_tjek_qty'");
        }else{
        mysql_query("INSERT INTO balance_of_order (id, product, amount) VALUES ('','$product','$product_tjek_qty')");
        }

        $order_updated = true;
        }
        }