PHP附件下载中文名称乱码的解决方法

$filename = urlencode("下载文档");
header ( "Content-disposition: attachment; filename=$filename.xls" );

$ua = _SERVER["HTTP_USER_AGENT"];
$filename = "中文 文件名.txt";
$encoded_filename = urlencode($filename);
$encoded_filename = str_replace("+", "%20", $encoded_filename);
header('Content-Type: application/octet-stream');
if (preg_match("/MSIE/", $ua)) {
  header('Content-Disposition: attachment; filename="' . $encoded_filename . '"');
} else if (preg_match("/Firefox/", $ua)) {
  header('Content-Disposition: attachment; filename="utf8\'\'' . $filename . '"');
} else {
  header('Content-Disposition: attachment; filename="' . $filename . '"');
}