Client Login

Was gonna try to remake my client login section, preferably with PHP and MySQL, but I’m not entirely sure how to go around it - I don’t want to have to add any code to the works themselves, but I’d still like to a have a separate login for each folder and perhaps a master login.

Perhaps it’s not so much an administration page they log in to, but just their website that’s in progress or something.

Is there some way I can do this with some .htaccess or something?