Piwik\Menu\MenuAdmin::addDiagnosticItem PHP Method

    public function addDiagnosticItem($menuName, $url, $order = 50, $tooltip = false)
    {
        $this->addItem('CoreAdminHome_MenuDiagnostic', $menuName, $url, $order, $tooltip);
    }

 public function configureAdminMenu(MenuAdmin $menu)
 {
     $userPreferences = new UserPreferences();
     $default = $userPreferences->getDefaultWebsiteId();
     $idSite = Common::getRequestVar('idSite', $default, 'int');
     if (Piwik::isUserHasAdminAccess($idSite)) {
         $menu->addDiagnosticItem('Custom Variables', $this->urlForAction('manage'), $orderId = 20);
     }
 }