The Zend_Auth_Adapter_OpenId adapter can be used to authenticate users using remote OpenID servers. This authentication method assumes that the user submits only their OpenID identity to the web application. They are then redirected to their OpenID provider to prove identity ownership using a password or some other method. This password is never provided to the web application.

The OpenID identity is just a URI that points to a web site with information about a user, along with special tags that describes which server to use and which identity to submit there. You can read more about OpenID at the » OpenID official site.

The Zend_Auth_Adapter_OpenId class wraps the Zend_OpenId_Consumer component, which implements the OpenID authentication protocol itself.

Note: Zend_OpenId takes advantage of the » GMP extension, where available. Consider enabling the GMP extension for better performance when using Zend_Auth_Adapter_OpenId.

As is the case for all Zend_Auth adapters, the Zend_Auth_Adapter_OpenId class implements Zend_Auth_Adapter_Interface, which defines one method: authenticate(). This method performs the authentication itself, but the object must be prepared prior to calling it. Such adapter preparation includes setting up the OpenID identity and some other Zend_OpenId specific options.

However, as opposed to other Zend_Auth adapters, Zend_Auth_Adapter_OpenId performs authentication on an external server in two separate HTTP requests. So the Zend_Auth_Adapter_OpenId::authenticate() method must be called twice. On the first invocation the method won't return, but will redirect the user to their OpenID server. Then after the user is authenticated on the remote server, they will be redirected back and the script for this second request must call Zend_Auth_Adapter_OpenId::authenticate() again to verify the signature which comes with the redirected request from the server to complete the authentication process. On this second invocation, the method will return the Zend_Auth_Result object as expected.

The following example shows the usage of Zend_Auth_Adapter_OpenId. As previously mentioned, the Zend_Auth_Adapter_OpenId::authenticate() must be called two times. The first time is after the user submits the HTML form with the $_POST['openid_action'] set to "login", and the second time is after the HTTP redirection from OpenID server with $_GET['openid_mode'] or $_POST['openid_mode'] set.

You may customize the OpenID authentication process in several way. You can, for example, receive the redirect from the OpenID server on a separate page, specifying the "root" of web site and using a custom Zend_OpenId_Consumer_Storage or a custom Zend_Controller_Response. You may also use the Simple Registration Extension to retrieve information about user from the OpenID server. All of these possibilities are described in more detail in the Zend_OpenId_Consumer chapter.