I am looking to allow users to register and login using their social network accounts from:
OpenID seems like a good option but it doesn't appear that Facebook or Twitter support it which kind of makes it useless. Any suggestions on a free (for large sites) option? OAuth seems good, but isn't that going to require the user to authenticate everytime? I want something more automated so they click 'twitter' and if they are logged in on twitter and have already granted authorization, they get automatically logged in on our site. I can handle the work on my site and have no probelm working with apis.
Thanks!
The OpenID Wiki lists multiple OpenId libraries for PHP (as well as other platforms), not being a PHP developer, but someone who is doing something very similar this should help you.
One other thing to bear in mind is that both Facebook and Twitter use the related OAuth standard and require you to register your application with them before you're able to use them as authentication providers.
You can find more information on using Facebook to authenticate your users here.
Hope this helps you.