most efficient way to define site root

I'm creating a quasi-cms, and I'm trying to determine if I should just call the includes based on whatever directory I'm in (i.e. ../includes/header.php and 1 deeper ../../includes/header.php) which seems like a poor solution, or if there is a global variable, or a simple function that would allow me to determine my site root and toss it in front of my includes.

Come on Hoff, I know you're out there

 

the variable: _SERVER["DOCUMENT_ROOT"]

will give you the root directory.

 

Thaaaat's the one. Thank you very much.