Before you can extend the sample, you must first register your application as described in Getting Your Application ID for Web Authentication. After you register your application, substitute your own application ID and secret key for the default values specified in the Application-Key.xml configuration file in the sample application.
Now you're ready to explore ways to develop the sample code further to meet your site's needs. Look through the PHP library in the file named WindowsLiveLogin.php to see what additional functionality is available to you. You can also read the topics in the Web Authentication Reference section for even more information about how Web Authentication works.
You can also customize the sign-in and registration pages that your users experience. For more information, see Customizing Sign-in and Registration.
Finally, you can share your discoveries and learn from others by participating in the Windows Live ID Development Forum. See you there!