php 关于readfile下载大文件失败的解决方法

if (file_exists($file))
{
  if (FALSE!== ($handler = fopen($file, 'r')))
  {
    header('Content-Description: File Transfer');
    header('Content-Type: application/octet-stream');
    header('Content-Disposition: attachment; filename='.basename($file));
    header('Content-Transfer-Encoding: chunked'); //changed to chunked
    header('Expires: 0');
    header('Cache-Control: must-revalidate, post-check=0, pre-check=0');
    header('Pragma: public');
    //header('Content-Length: ' . filesize($file)); //Remove
    //Send the content in chunks
    while(false !== ($chunk = fread($handler,4096)))
    {
      echo $chunk;
    }
  }
  exit;
}
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                ";