[PHP] It opens the file instead downloading

[Phpu]

Hi,
I have this script taken form php.net

$filename=plenar.mid;            
$file_to_download=files/plenar123.mid;  
  
$user_agent = strtolower ($_SERVER["HTTP_USER_AGENT"]);
header( "Content-type: application/force-download" );
if ((is_integer (strpos($user_agent, "msie"))) && (is_integer (strpos($user_agent, 
"win")))) {
  header( "Content-Disposition: filename=".$filename);
} else {
  header( "Content-Disposition: attachment; filename=".$filename);
}
header( "Content-Description: File Transfert");
@readfile($file_to_download);

Instead downloading the file the script opens it.
I wanna save the file to disk not open it. How should i do it?
Thanks