how do you get to the root of your site using php?

[subduck]

if i'm this point of location:
www.subduck.com/tutorials/subclassing.php

how do i get here:
www.subduck.com

without using the following links:
www.subduck.com
../../

you can't use $SERVER_NAME because this would be the outcome:
www.subduck.com/www.subduck.com


please help me :(