PHP-MySQL help needed.
I'm learning now some PHP to store data in a database (MySQL) and now I want to develop a pseudo-chat. What I want to do is to display which users are on-line and a chat window. What I don't know how to do is that if a user leaves the chat closing the window how can I erase that user from the people who is on-line, or drop him from the table that stores the current users?