This honestly shouldn't be very hard for you to do at all. Store each client's information in a database (user|pass|directory)....then write a script (again, not hard) that connects to the database, then verifies that user/pass is correct. If it is, then redirect to their directory.
AND, you really don't have to have a directory column in there either. YOu can simply store user/pass in database. If pass is valid for user, then redirect to /user.
http://programmingtalk.com/showthread.php?t=10921
There is a basic example that redirects to /what username the person entered. all you would have to do is add an additional check to make sure that the user is in the database, etc... and then redirect. This isn't horribly secure, because anyone could just type in /user, but then of course, you could get into sessions, etc....but the example on that thread is very basic, but it does what you want.
Kevin
PS. Sorry for the long winded reply