i add a column 'a' to zen_orders talbe
i add $a = something; to tpl_checkout_confirmation_default.php,

i want the $a automatically inserted into zen_orders table when an order placed,

so i go to classes/order.php, find the codes before zen_db_perform(TABLE_ORDERS, $sql_data_array); i tried adding
'a' => $a

but the value of $a is not inserted into zen_orders talbe

is there anyone who knows well in zencart could give me any idea?