choosing the name to save a file as in a script

Hi everyone, I've seen something very interesting on some site that would allow me to use PHP-authentification to allow download of certain files.... Here's what the link on the site looks like: http://www.whatever.com/view.php3/filename.ext?id=17982 Now, the script that sends the file is view.php3. What this does is get a file with id 17982, either by reading it from a database or something (not important) and sends it back to the browser and the browser thinks it is actually file "filename.ext"... What I don't get is how can you pass "/filename.ext?id=17982" as an argument? I tried it and the web server will of course look for file "filename.ext", but it doesn't exist since view.php3 is obviously not a directory... Any ideas how this can be achieved? I thought it could be some kind of 404 error redirection to a php script, but then all bad links on the whole site would be sent to that script and they don't appear to be (at least I get a normal 404 error everywhere else) Thanks for your time