server side path to filesystem How do I get the server side directory of the filesystem path?

For example, my files are in http://mydomain.local/myfiles; this is the root of my drupal filesystem. I am searching for a function that will give me d:\xampp\htdocs\drupal\myfiles.
$_SERVER['DOCUMENT_ROOT'] will get you the d:\xampp\htdocs and $base_path or base_path() will give you the Drupal part.