php 檔案下載-檔案中文名稱顯示亂碼處理

<?php
  $hua = $_SERVER['HTTP_USER_AGENT'];
 
  $showname = "鱷魚頭上一隻蝴蝶.jpg";
  $encode_zh = urlencode($showname);
  $encode_zh = str_replace("+","%20",$encode_zh);
  $on_disk = "1.jpg";
  $file = fopen($on_disk,"r"); // 開啟檔案
  //ob_end_clean();
  header("Content-type: application/octet-stream");
  header("Accept-Ranges: bytes");
  header("Accept-Length: ".filesize($on_disk));
  if (preg_match("/MSIE/",$hua))
  {
   header('Content-Disposition: attachment; filename="'.$encode_zh);
  }elseif (preg_match("/Firefox/",$hua))
  {
   header('Content-Disposition: attachment; filename*="utf8/'/''.$showname);
  }else
  {
   header('Content-Disposition: attachment; filename="'.$showname.'"');
  }
  $download= fread($file, filesize($on_disk));
  echo $download;
  fclose($file);
?>