AUTH_USER request variable. This will be empty if your web app allows anonymous access, but if your server’s using basic or Windows integrated authentication, it will contain the username of the authenticated user.
In an Active Directory domain, if your clients are running Internet Explorer and your web server/filesystem permissions are configured properly, IE will silently submit their domain credentials to your server and
AUTH_USER will be
MYDOMAIN\user.name without the users having to explicitly log in to your web app.