title: how to prevent user from using multiple browsers at a time
body: <p>I have login system in my site and users have to pay for using my site. As they have to pay, I am afraid that one user may share his username and password with another users. So I have to set my login system in such way that no user can use more than one browser at a time. But they can change the browser from time to time (not at the same time but different periods of a day). How can I implement that by php? Any idea?</p>