PHP download help

I have a php problem. Im no good at it… I
have bought a force download app from FlashDen. I cant get in hold of the authour, so i hope someone else can help me out here.
It´s regarding the path of the files to be downloaded from, in the php file. As it is now, I have to put all my download files out in the root or in the same folder as i put the download.php file in.
I have a folder named “images_down” and i want the path to go there.
here is the php code as original:

    $file = $_GET["f"];
    header("Content-type: application/octet-stream");
    header("Content-Disposition: attachment; filename=\"$file\"
    $fp = fopen("$file", "r");

So how to put in ánd where my “images_down” folder??
Hope someone can help…
Best regardn and new year to your all //Hrbi