Force download a image

To force download an image and not to be opened in the browser reaches the following script:

sample code:
<?PHP
$file = "test.jpg";
header("Pragma: public");
header("Expires: 0");
header("Cache-Control: must-revalidate, post-check=0, pre-check=0");
header("Content-Type: application/octet-stream");
header("Content-Disposition: attachment; filename=$file");
header("Content-Transfer-Encoding: binary");
readfile( $file);
exit;
?>

Caution! with the script, a visitor can also download PHP files etc..
If you get the variable from a form, you need to install a lock
<?PHP
$file = "test.jpg";
$allow_files = Array(".jpg", ".gif", ".png");
if (!strpos($file,$allow_files))
{
header("Pragma: public");
header("Expires: 0");
header("Cache-Control: must-revalidate, post-check=0, pre-check=0");
header("Content-Type: application/octet-stream");
header("Content-Disposition: attachment; filename=$file");
header("Content-Transfer-Encoding: binary");
readfile( $file );
exit;
}
?>