如何在浏览器直接下载文件而不输出

function force_download($filename = '', $data = '', $set_mime = FALSE)
   {
      if ($filename === '' OR $data === '')
      {
         return;
      }
      elseif ($data === NULL)
      {
         if ( ! @is_file($filename) OR ($filesize = @filesize($filename)) === FALSE)
         {
            return;
         }

         $filepath = $filename;
         $filename = explode('/', str_replace(DIRECTORY_SEPARATOR, '/', $filename));
         $filename = end($filename);
      }
      else
      {
         $filesize = strlen($data);
      }

      // Set the default MIME type to send
      $mime = 'application/octet-stream';

      $x = explode('.', $filename);
      $extension = end($x);

      if ($set_mime === TRUE)
      {
         if (count($x) === 1 OR $extension === '')
         {
            /* If we're going to detect the MIME type,
             * we'll need a file extension.
             */
            return;
         }

         // Load the mime types
         $mimes =& get_mimes();

         // Only change the default MIME if we can find one
         if (isset($mimes[$extension]))
         {
            $mime = is_array($mimes[$extension]) ? $mimes[$extension][0] : $mimes[$extension];
         }
      }

      /* It was reported that browsers on Android 2.1 (and possibly older as well)
       * need to have the filename extension upper-cased in order to be able to
       * download it.
       *
       * Reference: http://digiblog.de/2011/04/19/android-and-the-download-file-headers/
       */
      if (count($x) !== 1 && isset($_SERVER['HTTP_USER_AGENT']) && preg_match('/Android\s(1|2\.[01])/', $_SERVER['HTTP_USER_AGENT']))
      {
         $x[count($x) - 1] = strtoupper($extension);
         $filename = implode('.', $x);
      }

      if ($data === NULL && ($fp = @fopen($filepath, 'rb')) === FALSE)
      {
         return;
      }

      // Clean output buffer
      if (ob_get_level() !== 0 && @ob_end_clean() === FALSE)
      {
         @ob_clean();
      }

      // Generate the server headers
      header('Content-Type: '.$mime);
      header('Content-Disposition: attachment; filename="'.$filename.'"');
      header('Expires: 0');
      header('Content-Transfer-Encoding: binary');
      header('Content-Length: '.$filesize);
      header('Cache-Control: private, no-transform, no-store, must-revalidate');

      // If we have raw data - just dump it
      if ($data !== NULL)
      {
         exit($data);
      }

      // Flush 1MB chunks of data
      while ( ! feof($fp) && ($data = fread($fp, 1048576)) !== FALSE)
      {
         echo $data;
      }

      fclose($fp);
      exit;
}