best way to properly build an include path *regardless* from where I am calling the include?

I am confusing myself reading the docs just now. i.e.: include_path basename() and dirname() I had thought from many months ago that <?php include '/somedir/somefile.php'; ?> would include somefile.php living in somedir regardless from where in the site structure I am calling it. Now it does not seem to be doing that. What is the safest (portable) way to properly build an include path *regardless* from where in my site I am calling the include? If I understand right I can put includes in a dir/ specified by the include_path setting, but I also want to know how to do this from just within the root of my virtual server. -Govinda