$_SERVER['AUTH_USER'], or sometimes it exists as $_SERVER['PHP_AUTH_USER']. I think whichever exists for you is dependent upon your version of PHP, but I can't tell you which exists for which version(s).
I guess it helps if you use the right one. So for anyone doing this, always create a global variable at the begining of the script that defins which of these variables you're version uses. So you don't have to go through every page and change them all everytime you switch over to another version of php.