ho trovato la soluzione:
$fname = "nomedelfile.estensione";
$fpath = "percorso/nomedelfile.estensione";

header("Content-Type: application/octet-stream; type=ctr");
header("Content-Type: application/force-download");
header("Content-Type: application/download");
header("Content-Disposition: attachment; filename=$fname");
header("Content-Transfer-Encoding: binary");
readfile($fpath);
exit (0);

grazie ciao