/**
  * Returns a cache identification string.
  *
  * @return string A string to be used as part of keys.
  */
 protected function get_cache_identifier()
 {
     $identifiers = array();
     if ($this->selectedsharingoption & self::SHARING_ALL) {
         // Nothing to do here.
     } else {
         if ($this->selectedsharingoption & self::SHARING_SITEID) {
             $identifiers[] = cache_helper::get_site_identifier();
         }
         if ($this->selectedsharingoption & self::SHARING_VERSION) {
             $identifiers[] = cache_helper::get_site_version();
         }
         if ($this->selectedsharingoption & self::SHARING_INPUT && !empty($this->userinputsharingkey)) {
             $identifiers[] = $this->userinputsharingkey;
         }
     }
     return join('/', $identifiers);
 }