Our PHP site uses a home-grown forms auth implementation. We're considering making our site into an OpenID provider so that we can authenticate (using our exisitng user accounts) users on a StackExchange site. If we have to add code or UI to our login form or add fields to our database, that's fine-- we just don't want to have to rip out what we have and replace it all in order to get OpenID provider support.
There are several libraries for PHP linked on the OpenID website. Anyone have experience with using any of these libraries to adding OpenID provider capability to a PHP site, and if so which is your favorite and why? Are there particular libraries you would stay away from, and if so, why?
Well, most of the options listed on that page don't have provider support, so I think your options come down to OpenID Enabled's php-openid or Zend_OpenId. I'm partial to the openidenabled libraries myself, but then again, I was on the team that wrote them. php-openid is generally compatible with a wider range of PHP versions, and its test coverage is pretty extensive.
I don't really know much about the Zend offering, but it might be appropriate if you're fortunate enough to not need PHP 4 compatibility and like that framework thing they do.