INSERT & UPDATE

I am trying to UPDATE a column after an INSERT, and getting a NULL result,
let me show you some code... This is from a PHP page by the way

$sql="BEGIN WORK; INSERT INTO checking (check_date, check_amount,
check_number, check_payee, check_trans_type) VALUES ('$check_date',
'$check_amount', '$check_number', '$check_payee', '$check_trans_type');
UPDATE checking SET check_post_balance = (check_pre_balance -
$check_amount::money); COMMIT WORK;";

After the INSERT, I am trying to update the check_post_balance column to
check_pre_balance MINUS what the check_amount was. I have tried many
variations on this and all I get are NULL results. Can anyone see what in
the heck I am doing wrong? :) Thanks in advance!

Preston