Using PHP Headers to Force Download

<?PHP
// Define the path to file
$file = 'ryboe_tag_cloud.zip';

if(!file)
{
    // File doesn't exist, output error
    die('file not found');
}
else
{
    // Set headers
    header("Cache-Control: public");
    header("Content-Description: File Transfer");
    header("Content-Disposition: attachment; filename=$file");
    header("Content-Type: application/zip");
    header("Content-Transfer-Encoding: binary");
   
    // Read the file from disk
    readfile($file);
}
?>

SOURCE:

http://www.ryboe.com/tutorials/php-headers-force-download