Horde_Kolab_Storage_Cache::requireParameter PHP Method

    public function requireParameter($parameters, $type, $key)
    {
        if (!isset($parameters[$key])) {
            throw new Horde_Kolab_Storage_Exception(sprintf('Unable to determine the %s cache key: The "%s" parameter is missing!', $type, $key));
        }
    }

 /**
  * Compose the list key.
  */
 private function _setListId()
 {
     foreach (array('host', 'port', 'user') as $key) {
         $this->_cache->requireParameter($this->_parameters, 'list', $key);
     }
     ksort($this->_parameters);
     $this->_list_id = md5(serialize($this->_parameters));
 }