public static function say_size($bytes, $unit = 'B', $stop = false): string
  {
// pretty printer for byte values
//
    $i = 0;
    $units = ['', 'K', 'M', 'G', 'T'];
    while ( $stop || ($bytes > 2000) ){
      $i++;
      $bytes /= 1024;
      if ( $stop === $units[$i] ){
        break;
      }
    }
    return sprintf("%5.2f %s".$unit, $bytes, $units[$i]);
  }