function filesize_format($bytes, $format = '', $force = ''){
  $force = strtoupper($force);
  $defaultFormat = '%01d %s';
  if (strlen($format) == 0)
  $format = $defaultFormat;
  $bytes = max(0, (int) $bytes);
  $units = array('B', 'KB', 'MB', 'GB', 'TB', 'PB');
  $power = array_search($force, $units);
  if ($power === false)
  $power = $bytes > 0 ? floor(log($bytes, 1024)) : 0;
  return sprintf($format, $bytes / pow(1024, $power), $units[$power]);
}
?>

  echo filesize_format(1024) . "n";
  echo filesize_format(500000, '%01.5lf %s', 'mb') . "n";
  echo filesize_format(1023) . "n";
  echo filesize_format(1023, '', 'kb') . "n";
  echo filesize_format(1048576) . "n";
  echo filesize_format(1073741824, '', 'b') . "n";
?>