PHP大文件下载的奇怪问题

$filePath  = getfilepath(); //absolute

header('Content-type: application/octet-stream;');

header('Content-Disposition: attachment; filename="' . utf8_decode($fileName) . '"');

$size = getfilesize();//return string

header('Content-length: ' . $size);

if ($fd = fopen($filePath, 'rb')) {

  session_write_close();

  while (!feof($fd)) {

    print (fread($fd, 65535));

  }

  fclose($fd);

}