I have made a website on which a user has to log in using a username and password. I have seen these in so many website, we can connect through facebook account. And I want to add the same to my web site. How is it possible,using php sdk or the javascript sdk?
The team at Facebook has spent considerable time documenting this, I would suggest you start there:
http://developers.facebook.com/docs/guides/web#login
http://developers.facebook.com/docs/authentication/