/**
  * 打印日志
  * @param BigpipeLogSeverity $severity
  * @param array $args
  */
 private static function log($severity, $args)
 {
     if (self::$_severity < $severity) {
         // 跳过低级别
         return;
     }
     $log_str = null;
     $num_args = count($args);
     if (1 < $num_args) {
         $format = array_shift($args);
         $log_str = vsprintf($format, $args);
     } else {
         if (1 == $num_args) {
             $log_str = vsprintf('%s', $args[0]);
         } else {
             $log_str = "[empty log args]";
         }
     }
     // [严重性][时间]
     $out_str = sprintf("[%s]%s\n", BigpipeLogSeverity::to_string($severity), $log_str);
     self::$_last_error_message = $out_str;
     //         else
     //         {
     //             fwrite(self::$_logfile, $out_str);
     //         }
 }