Weird $user variable confusion between drupal 6 and ubercart - drupal-6

Question

I've set up a hook alter that adds a submit function to a webform submit chain. In that function alongside some other processing I add:
function webform_checkout_form_alter(&$form, &$form_state, $form_id) {
$form['#submit'][] = 'webformSubmitAlteration';
}
function webformSubmitAlteration(&$form, &$form_state) {
global $user;
$user->last_submission = $sids[1];
drupal_set_message('<pre> current user is: '. check_plain(print_r($user, TRUE)) .'</pre>');
}
That function also redirects to the ubercart checkout page.
The printed $user shows as (edited for clarity):
current user is: stdClass Object
(
[uid] => 1
[name] => paul.dambra
[last_submission] => 85
)
I then process the cart through checkout which fires an ubercart conditional action that runs some custom php. Basically when this payment completion is caught I want to use the last_submission value to switch the submitted form from draft to complete so that my users now which applications are done and dusted.
In that code I have added:
global $user;
drupal_set_message('<pre> user is : '. check_plain(print_r($user, TRUE)) .'</pre>');
$sid = $user->last_submission;
drupal_set_message('<pre> sid is : '. check_plain(print_r($sid, TRUE)) .'</pre>');
And when we see the output of those drupal_set_message calls:
user is : stdClass Object
(
[uid] => 1
[name] => paul.dambra
[last_submission] => 47
)
I know that 47 is the last_submission value from prior to processing the form in question. So even though when I submit the form I can see the correct value for last_submission when I immediately afterwards process the cart I see an old value.
Any pointers on what I might be doing wrong would be super!

Answer 1

I had to edit my code to:
global $user;
if ($user) {
$array = array('last_submission' => $sids[1]);
user_save($user, $array);
}
It now works as I expect. Hope that helps someone else...