Hi hbk,
Obviously, you are on the first step to making a XAMP based web application. Welcome
A quick and dirty method is to have a authentication mechanism in b.php, which will check for the username and password submitted in form of a.php. Once the user is authenticated, then you can set a session variable userid = <user_id>.
Now, every page after the authentication takes place, you need to check one thing, <user_id> is present in session or not. If it is, then the user is authenticated, if it isnt, then the user is not authenticated (redirect to login).
Kapische?