stackoverflow's implementation of openid for PHP

How do I implement the same login system as stackoverflow?

Where anyone can login based on their existin开发者_如何学JAVAg username/passwords from other web applications like, google, paypal, facebook, twitter and much much more?

For .NET you may checkout DotNetOpenAuth, that's what SO uses to implement OpenId authentication. So add the NuGet and get started. And for the nice icons checkout openid-selector.

For PHP you may checkout this project.

There are many OpenId solutions. Here is a list.

http://openid.net/add-openid/add-getting-started/

http://openid.net/developers/libraries/

For nice looking UI as Darin Dimitrov said you need to go with

http://code.google.com/p/openid-selector/