Same login on 2 websites

I have a web server with the main php application in it and a folder with another different web app in it. They were developed separately, so each one connects to a different database and has different login forms.

My question is: if i am on the web app on the folder, how can I connect to the database of the first web app just to login and then connect back to the "folder web app" database to retrieve the rest of the information?

Sorry If I did not express my self well. I am not looking for a php script done for me, i just need some info on how i can do it

Thanks!