Simple module to add a header with the user ID

drupal_uid_header.module

<?php
/**
 * @file   drupal_uid_header.module
 * @author António P. P. Almeida <appa@perusio.net>
 * @date   Tue Dec 27 2011
 *
 * @brief Sets an HTTP header with the UID (Drupal 6).
 *
 */

/**
 * Implements hook_init().
 */
function drupal_uid_header_init() {
  global $user;

  if ($user->uid > 0) {
    $h = header_list();
    $k = array_search('X-Drupal-UID', $h);
    // Check to see if the the current user is valid, i.e., not tampered with.
    if ($k) {
      $current_uid_header_array = explode(' ', $h[$k]);
    }
    // If invalid or not set then send the header with the correct UID.
    if (!$k || $current_uid_header_array[1] != $user->uid) {
       drupal_add_http_header('X-Drupal-UID', filter_var($user->uid, FILTER_SANITIZE_NUMBER_INT));
    }
  }
} // drupal_uid_header_init