How to populate the REMOTE_USER in Apache Server from Active Directory (SSO related)
The first step to this involves a change through group policy to add your Joomla site to the Intranet security group in internet explorer. Google Chrome and Firefox respect the IE security group settings.
Once you have done this whenever your user browses your site from a domain authenticated computer the browser will automatically send a user header with the already authenticated username, this can be accessed in PHP using the $_SERVER['PHP_AUTH_USER'] superglobal.