[development] Getting absolute directory path

[Darren Oh]

Some users keep contributed modules outside the Drupal directory and  
create symlinks to them in sites/all/modules. This causes problems  
for Chat Room, which uses a PHP file in its own directory for AJAX  
updates (to avoid loading all of Drupal on each update) but must  
include some files from Drupal. I was using the following code to  
find the Drupal files:

     $chatroom_base = urldecode($_POST['chatroom_base']);
     $depth = substr_count($chatroom_base, '/');
     chdir(str_repeat('../', $depth+1));

If people are keeping Chat Room outside of Drupal, chdir() will need  
the absolute path to the Drupal directory. What is the best way to  
get this?