private static function insert_source_folder($source_id = '', $folder_id = '')
 {
     global $wpdb;
     if (empty($source_id) || empty($folder_id)) {
         return '';
     }
     $return = false;
     $source = DLN_Term_Helper::select_source_folder($source_id);
     if (isset($source['folder_id']) && !empty($source['folder_id'])) {
         $return = $wpdb->update($wpdb->dln_source_folder, $data, array('source_id' => $source_id, 'folder_id' => $folder_id));
     } else {
         $return = $wpdb->insert($wpdb->dln_source_folder, array('source_id' => $source_id, 'folder_id' => $folder_id));
     }
     return $return;
 }