Phil Sturgeon's Blog: Hijacking Headers to Force Downloads
The question [I posed on Twitter] was: "How to force a download of any file of any type, not on your server, without Apache tweaking? Images are displaying and need em to download." Essentially, I wanted to be able to link to a file that was not on the server in question and anywhere in the world, which could be of any size, any media type and could be potentially very high traffic.
Answers varied from using readfile to just letting the browser handle it. None of the responses were quick right until he came across one that recommended some settings in an .htaccess file. It uses mod_rewrite (Apache) to redirect the user to a new resource while adding a "Content-Disposition" header in the process (of "attachment").