make all the menus dropdown (not only main menu)

I'm trying with this modification in template.php:

function tweme_preprocess_page(&$vars) {
  $page = $vars['page'];
 
  // Build navbar menu.
  $menu_id = variable_get('menu_main_links_source', 'main-menu');
  $usermenu_id = variable_get('menu_main_links_source', 'user-menu');
  $menu_tree = menu_tree($menu_id);
  $usermenu_tree = menu_tree($usermenu_id);
  $vars['navbar_menu'] = drupal_render($menu_tree);
  $vars['navbar_user'] = drupal_render($usermenu_tree);

and this in page.tpl.php:

if ($user_menu):

print $user_menu

endif

if ($main_menu):

print $main_menu

endif

But its not working. Can anybody please help me?

thanks a lot.

I would like to be able to put my different menu blocks in the topbar and make them dropdown. I think this would be a cool feature to implement.

thanks.