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?

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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.

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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/