You will need to pass the file name, path and disposition name to the browser as headers, follow the below piece:
if ($startdownload == "yes") {
header("Content-type: application/x-ms-download;name=$file_location_url");
// or if this doesnt force a download box in IE, use
header("Content-type: application/force-download;name=$file_location_url");
// the below header allows you to name the file when downloading
header("Content-Disposition: attachment; filename=$download_filename");
exit;
}
This should be enough for the download box to be forced.