This article is a good start: http://devzone.zend.com/1254/getting-started-with-openid-and-php/
I did it once, for a project in which I adopted Janrain solution (most because I could not use the PEAR package): http://www.janrain.com/
This plugin remembers the user (even after logout) and when he comes back gives a suggestion.
In that project, users had the ability to create a standard account or to log with an available OpenID (Flickr, Twitter, Yahoo, Google, Windows Live and MySpace).
If that was first login from openID and there was no match with a standard accounts, then it was created a new account. Otherwise, the user was suggested to merge it with the matched account. Anyway, if the user had also a standard account he had the ability to merge the account.
A (requested) nightmare :S
Virtually, in that project, a user can login with many OpenIDs and link always to the same standard account.
To fully enable an account created with an OpenID, it was requested to complete the login inserting an email, usually from the OpenId service you get enough data but sometimes, as with Twitter, you don't get any email address. So, for this reason, it was requested.
Once you have an email and the user choose to login with Facebook and Twitter and Google and whatever else, when he completes with the same email you can merge the accounts..
I repeat: a requested nightmare :'(
Anyway, if you want to check, through janrain plugin you make a request to their api and they reply with an identifier and some data, here there is an example:
http://documentation.janrain.com/social-login-widget/user-s-guide/server-side-retrieval
My suggestion is to keep it simple if you can. o_o'
