public function MessageGetFoldersById($folder_id)
 {
     // $folder_id : string or string array
     $this->CheckAndSetHeader(__FUNCTION__);
     $reg_args = $this->getRegularArgs($folder_id, __FUNCTION__);
     $this->encodeString($reg_args);
     $results = parent::MessageGetFoldersById($reg_args);
     $this->methodClose();
     return $this->decodeString($results->folder);
 }