IIS server variables problem

My company has a IIS server which is hosting a PHP intranet application. The IIS basically authenticates the user by using Windows Integrated Authentication so once the user is logged on his/her machine he doesnt have to provide credentials again. One of the employees is using Remote Desktop to logon to his Office workstation and AUTH_USER and REMOTE_USER server variables are returning the computer name instead of his Windows Login ID. I'm totally confused about this one. I will highly appreciate if anyone can tell me what's wrong here.