On the AOL Developer Network Jack Herrignton has posted an introduction to OpenID and how you can use it in your PHP application.

OpenID is another one of those "cool technologies you've never heard of" type of deals. OpenID is an open source initiative that provides a way for Web users to register their identity in one place and then use that identity anywhere on the Web that supports OpenID. [...] Before getting an OpenID, though, I should point out that it’s not required that you run an OpenID provider to OpenID-enable your application. In fact, no special software or service is required in your Web application. The only requirement is that your Web server programming language be able to make HTTP requests of the OpenID provider, which is something all of them can do.

Once you have your OpenID account set up, his simple script, making use of the ability that any version of PHP has to open URLs, can authenticate off of a remote OpenID service - that has the cURL extension compiled in.