Delivers a "page not found" error to the browser.
Page callback functions wanting to report a "page not found" message should return MENU_NOT_FOUND instead of calling drupal_not_found(). However, functions that are invoked in contexts where that return value might not bubble up to menu_execute_active_handler() should call drupal_not_found().
function drupal_not_found() {
drupal_deliver_page(MENU_NOT_FOUND);
}