Custom coding: Adding advanced validation or submit code

An example for your particular use-case would look like the following:

- Make a new directory in sites/all/modules called "mywebform_extra".
- Make a new text file in sites/all/modules/mywebform_extra called mywebform_extra.info. Put this inside of it:

name = Webform Extra
description = Customizations for the Webform module.
core = 6.x
package = Webform
dependencies[] = webform

- Make a new text file in sites/all/modules/mywebform_extra called mywebform_extra.module. Put this inside of it:

/**
 * Implementation of hook_form_alter().
 */
function mywebform_extra_form_alter(&$form, &$form_state, $form_id) {
  // Add validation for a particular Webform node:
  if ($form_id == 'webform_client_form_44') {
    // Simply add the additional validate handler.
    $form['#validate'][] = 'mywebform_extra_validate_44';

    // Add the submit handler after the existing Webform submit handler,
    // but before the second Webform handler. Pop off the first one and add
    // ours second.
    $first = array_shift($form['#submit']);
    array_unshift($form['#submit'], $first, 'mywebform_extra_submit_44');
  }
}

/**
 * Validation handler for Webform ID #44.
 */
function mywebform_extra_validate_44(&$form, &$form_state) {
  global $user;
  if (!isset($user->roles[4])) {
    form_set_error('', t('Your user role is not allowed to submit this Webform.'));
  }
}

/**
 * Submit handler for Webform ID #44.
 */
function mywebform_extra_submit_44(&$form, &$form_state) {
  global $user;

  // Changes can be made to the Webform node settings by modifying this variable:
  $form['#node']->webform;

  // Insert things into other database tables or modify properties.
}

- Note that "webform_client_form_44" means modify the Webform form for the node with NID "44". Adjust this to match whichever webform node's form you're modifying.
- Turn on the new Webform Extra module you've just created.

Besides this tutorial, there is also an excellent reference at http://www.drupalcoder.com/blog/additional-processing-in-drupals-webform...