EDIT

/**

* Get the true path to the root of the Drupal site.

* Better than using DOCUMENT_ROOT and base_path().

*/

function absolute_path_to_drupal() {

    static $absolute_path_to_drupal = NULL;

    if ($absolute_path_to_drupal === NULL) {

        // Get the absolute path to this file:

        $dir = rtrim(str_replace('\\', '/', dirname(__FILE__)), '/');

        $parts = explode('/', $dir);

        // Iterate up the directory hierarchy until we find the website root:

        $done = FALSE;

        do{

            // Check a couple of obvious things:

            $done = is_dir("$dir/sites") && is_dir("$dir/includes") && is_file("$dir/index.php");

            if (!$done) {

                // If there's no more path to examine, we didn't find the site root:

                if (empty($parts)) {

                    $absolute_path_to_drupal = FALSE;

                    break;

                }

            // Go up one level and look again:

            array_pop($parts);

            $dir = implode('/', $parts);

            }

        }while (!$done);

        $absolute_path_to_drupal = $dir;

    }

    return $absolute_path_to_drupal;

}