static function sendNotification($a_action, $a_type, $a_wiki_ref_id, $a_page_id, $a_comment = null)
 {
     global $ilUser, $ilObjDataCache, $ilAccess;
     include_once "./Services/Notification/classes/class.ilNotification.php";
     include_once "./Modules/Wiki/classes/class.ilObjWiki.php";
     include_once "./Modules/Wiki/classes/class.ilWikiPage.php";
     $wiki_id = $ilObjDataCache->lookupObjId($a_wiki_ref_id);
     $wiki = new ilObjWiki($a_wiki_ref_id, true);
     $page = new ilWikiPage($a_page_id);
     // #11138
     $ignore_threshold = $a_action == "comment";
     // 1st update will be converted to new - see below
     if ($a_action == "new") {
         return;
     }
     if ($a_type == ilNotification::TYPE_WIKI_PAGE) {
         $users = ilNotification::getNotificationsForObject($a_type, $a_page_id, null, $ignore_threshold);
         $wiki_users = ilNotification::getNotificationsForObject(ilNotification::TYPE_WIKI, $wiki_id, $a_page_id, $ignore_threshold);
         $users = array_merge($users, $wiki_users);
         if (!sizeof($users)) {
             return;
         }
         ilNotification::updateNotificationTime(ilNotification::TYPE_WIKI_PAGE, $a_page_id, $users);
     } else {
         $users = ilNotification::getNotificationsForObject(ilNotification::TYPE_WIKI, $wiki_id, $a_page_id, $ignore_threshold);
         if (!sizeof($users)) {
             return;
         }
     }
     ilNotification::updateNotificationTime(ilNotification::TYPE_WIKI, $wiki_id, $users, $a_page_id);
     // #15192 - should always be present
     if ($a_page_id) {
         include_once "./Modules/Wiki/classes/class.ilObjWikiGUI.php";
         $link = ILIAS_HTTP_PATH . "/" . ilObjWikiGui::getGotoLink($a_wiki_ref_id, $page->getTitle());
     } else {
         include_once "./Services/Link/classes/class.ilLink.php";
         $link = ilLink::_getLink($a_wiki_ref_id);
     }
     include_once "./Services/Mail/classes/class.ilMail.php";
     include_once "./Services/User/classes/class.ilObjUser.php";
     include_once "./Services/Language/classes/class.ilLanguageFactory.php";
     include_once "./Services/User/classes/class.ilUserUtil.php";
     // see ilBlogPostingGUI::getSnippet()
     // see ilBlogPosting::getNotificationAbstract()
     include_once "Modules/Wiki/classes/class.ilWikiPageGUI.php";
     $pgui = new ilWikiPageGUI($page->getId());
     $pgui->setRawPageContent(true);
     $pgui->setAbstractOnly(true);
     $pgui->setFileDownloadLink(".");
     $pgui->setFullscreenLink(".");
     $pgui->setSourcecodeDownloadScript(".");
     $snippet = $pgui->showPage();
     $snippet = ilPageObject::truncateHTML($snippet, 500, "...");
     // making things more readable
     $snippet = str_replace('<br/>', "\n", $snippet);
     $snippet = str_replace('<br />', "\n", $snippet);
     $snippet = str_replace('</p>', "\n", $snippet);
     $snippet = str_replace('</div>', "\n", $snippet);
     $snippet = trim(strip_tags($snippet));
     // "fake" new (to enable snippet - if any)
     $current_version = array_shift($page->getHistoryEntries());
     $current_version = $current_version["nr"];
     if (!$current_version) {
         $a_type = ilNotification::TYPE_WIKI;
         $a_action = "new";
     }
     foreach (array_unique($users) as $idx => $user_id) {
         if ($user_id != $ilUser->getId() && $ilAccess->checkAccessOfUser($user_id, 'read', '', $a_wiki_ref_id)) {
             // use language of recipient to compose message
             $ulng = ilLanguageFactory::_getLanguageOfUser($user_id);
             $ulng->loadLanguageModule('wiki');
             $subject = sprintf($ulng->txt('wiki_change_notification_subject'), $wiki->getTitle(), $page->getTitle());
             $message = sprintf($ulng->txt('wiki_change_notification_salutation'), ilObjUser::_lookupFullname($user_id)) . "\n\n";
             if ($a_type == ilNotification::TYPE_WIKI_PAGE) {
                 // update/delete
                 $message .= $ulng->txt('wiki_change_notification_page_body_' . $a_action) . ":\n\n";
                 $message .= $ulng->txt('wiki') . ": " . $wiki->getTitle() . "\n";
                 $message .= $ulng->txt('page') . ": " . $page->getTitle() . "\n";
                 $message .= $ulng->txt('wiki_changed_by') . ": " . ilUserUtil::getNamePresentation($ilUser->getId()) . "\n";
                 if ($snippet) {
                     $message .= "\n" . $ulng->txt('content') . "\n" . "----------------------------------------\n" . $snippet . "\n" . "----------------------------------------\n";
                 }
                 // include comment/note text
                 if ($a_comment) {
                     $message .= "\n" . $ulng->txt('comment') . ":\n\"" . trim($a_comment) . "\"\n";
                 }
                 $message .= "\n" . $ulng->txt('wiki_change_notification_page_link') . ": " . $link;
             } else {
                 // new
                 $message .= $ulng->txt('wiki_change_notification_body_' . $a_action) . ":\n\n";
                 $message .= $ulng->txt('wiki') . ": " . $wiki->getTitle() . "\n";
                 $message .= $ulng->txt('page') . ": " . $page->getTitle() . "\n";
                 $message .= $ulng->txt('wiki_changed_by') . ": " . ilUserUtil::getNamePresentation($ilUser->getId()) . "\n\n";
                 if ($snippet) {
                     $message .= $ulng->txt('content') . "\n" . "----------------------------------------\n" . $snippet . "\n" . "----------------------------------------\n\n";
                 }
                 $message .= $ulng->txt('wiki_change_notification_link') . ": " . $link;
             }
             $mail_obj = new ilMail(ANONYMOUS_USER_ID);
             $mail_obj->appendInstallationSignature(true);
             $mail_obj->sendMail(ilObjUser::_lookupLogin($user_id), "", "", $subject, $message, array(), array("system"));
         } else {
             unset($users[$idx]);
         }
     }
 }