PPL in 28 days is perfectly achievable. I know numerous people who have done it both in the UK and the USA and that includes all the ground study
The main factor is your commitment. You've got to be prepared to study most evenings and recap you previous lesson rather than chatting at the flying club etc etc but it can be done.
Why don't you give someone like the
The Great Circle :: Aviation Pilot Supplies & Flight Training
a call as he specializes in this sort of thing. His advice is free, he's a good instructor and an all round nice guy.