Chat Script that integrates with existing DB

I have an existing MySQL database that contains users names and passwords. I would like to add a chat script to the site - and would like users to log into this script - or if they are already logged into the site - using the same username and password that is in the MySQL database. Does anyone know of a chat script that will utilize existing MySQL usernames and passwords? How do you check to see if they are logged in already - so they dont have to enter the username and password again? Is this a seperate script?

Thanks