/**
  * Unmarks this guestbook entry.
  */
 public function unmark()
 {
     $markedEntries = UserGuestbookEntryEditor::getMarkedPosts();
     if (in_array($this->entryID, $markedEntries)) {
         $key = array_search($this->entryID, $markedEntries);
         unset($markedEntries[$key]);
         if (count($markedEntries) === 0) {
             UserGuestbookEntryEditor::unmarkAll();
         } else {
             WCF::getSession()->register('markedGuestbookEntries', $markedEntries);
         }
     }
 }