You should be able to change the header information. Here is an example:
function download($server_path){
$filename = $server_path;
$pos = strrpos($filename, "/") + 1;
$filename = substr($filename, $pos);
$mime_type = "binary/octetstream";
header("Content-Type: $mime_type");
header("Content-Disposition: attachment; filename=\"$filename\"");
header("Content-Transfer-Encoding: binary");
header("Content-Description: my file");
passthru("cat $server_path");
}