$file is the actual file (with the path included)
$new_filename is the filename that you want to send to the user. So when they download this will be the name used by the browser

the first 3 headers force the browser to download the file each and every single time, eg it's not pulling it from the memory/dish cache

the content type is just sent as some binary data type
the content-disposition tell the browser to force a download, do not open in the browser, even if you have a plugin to open it.

i use this script for images, word documents, pdfs and it works just fine.

[php]if (is_file($file))
{
header("Expires: Mon, 26 Jul 1997 05:00:00 GMT");
header("Cache-Control: no-store, no-cache, must-revalidate");
header("Pragma: no-cache");

header("Content-type: application/octet-stream");
header("Content-Disposition: attachment; filename=".$new_filename);
readfile($file);
exit();
}[/php]