Prova con questo.
Codice PHP:
// $file: percorso assoluto + nomefile del file da inviare al browser
function force_download($file)
{
$dim = filesize($file);
$nome_file = substr($file,strrpos($file,"/")+1,strlen($file));
$apptype = "application/force-download";
header("Content-Type: ".$apptype."; name=".$nome_file);
header("Content-Transfer-Encoding: binary");
header("Content-Length: ".$dim);
header("Content-Disposition: attachment; filename=".$nome_file);
header("Expires: 0");
header("Cache-Control: no-cache, must-revalidate");
header("Cache-Control: private");
header("Pragma: public");
readfile($file);
}