/**
  * add submenu box
  * @param $parent_slug
  * @param $url
  * @param $capability
  * @param string $page_title
  * @param string $menu_slug
  */
 public function add_submenu_box($parent_slug, $url, $page_title, $capability = 'manage_options', $menu_slug = '')
 {
     //valid
     if (empty($capability)) {
         $capability = 'manage_options';
     }
     if (empty($menu_slug)) {
         $menu_slug = sanitize_title($page_title) . '-' . $this->option('module_name');
     }
     $menu_id = HW_Module_Settings_page::add_menu_box($this->option('module_name'), array('title' => $page_title, 'page_title' => $page_title, 'capability' => $capability, 'menu_slug' => $menu_slug, 'link' => $url), $parent_slug);
     return $menu_id;
 }