i add a column 'a' to zen_orders table
i add $a = something; to checkout_confirmation page
i add some codes to classes/order.php
what do i make the value of $a write to database when an order placed?

i did a similar test successfully:
add a column 'a' to zen_orders table
add a text input to checkout_shipping page, set the default value = $a
add some codes to classes/order.php
so, the value of $a is written to database when an order placed

but what i want is
1. use this method to checkout_confirmation page
2. not a text input for customer, but the value $a = something; which from my checkout_confirmation file

anyone could help?