There’s really no reliable way to force the download of a file.
The best you could do, I think, would be to put this at the very top of each file (with no space above it):
<?php
header("Content-type: application/octet-stream");
header("Content-Disposition: attachment; filename=filename_of_your_choice");
?>
I think this would still generate a disturbing warning message in some browsers, depending on the security setting; and it should, if you think about it. For all the browser knows, It could be an executable trojan.
Also, users can customize a browser so that it does anything they want with files of various mime types so there’s really no way to "force" anything.