foo.php

<?php
// $Id$

/**
 * Implementation of hook_form_privatemsg_new_alter().
 */
function foo_form_privatemsg_new_alter(&$form, &$form_state) {
  $form['#submit'][] = 'foo_form_privatemsg_new_submit';
}

/**
 * Implementation of hook_menu_alter().
 */
function foo_menu_alter() {
  $callbacks['messages/user-name-autocomplete']['page callback'] = 'foo_messages_autocomplete_callback';
}

function foo_messages_autocomplete_callbac($string) {
}