PHP文件下载程序，怎样获取远程文件大小？跪求！

Question

if(file_exists($file))
{
//文件存在
$fp=fopen($file,'rb');
header("Pragma: public");
           header("Expires: 0");
           header("Cache-Control: must-revalidate, post-check=0, pre-check=0");
           header("Cache-Control: public", false);
           header("Content-Description: File Transfer");
Header("Content-type: application/octet-stream");
header("Accept-Ranges:bytes");
header("Content-Transfer-Encoding: binary");
header("Accept-Length:".filesize($file));
header("Content-Disposition: attachment; filename=".basename($file));
echo fread($fp,filesize($file));
fclose($fp);
exit();
}
else 
{
 echo("Requested file does not exist");
  die();
}