如何将OpenID添加到php站点？

I absolutely love StackOverflow's usage of OpenID for registration. How would I go about adding this to my site? It is PHP based. I went to openid.net and looked at the libraries there, but could find little in the way of documentation or examples.

This tutorial was helpful for a previous project I worked on using PHP and OpenID. It has an example app.

http://devzone.zend.com/article/3581

_{(source: zend.com)}

OpenID is great, but have one big disadvantage. Let's imagine that I'm having Google account and Yahoo account. I will login using Google and then I will try to login using Yahoo. And it will be different accounts; it's really difficult to remember what openID provider I was using for registration... So, be careful with OpenID on entertainment websites, but it is still ok for developers websites.

Check OpenId class from Zend Framework.

Documentation and examples.