A question on how to get the root directory

I am new to PHP having converted from ASP (don't beat me up, it just took me a while to see the light) MY QUESTION: What in PHP acts like the 'server.mappath' function in ASP? It gives you the ability to determain the root of the web site and use it as a reference. MY PROBLEM: I have been able to use '/some/folder/whatever.gif' to point to the image folder with GIFs inside no matter where I am in the site in the HTML part of my pages. It doesn't seem to work includes in the <?PHP ?> tags. I either have to use 'http://www.mysite.com/some/folder/whatever.inc' -OR- '../../whatever.inc'. Any ideas? INFO: I am running Apache with PHP4.03 installed as a module Thanx in advance!!