public function findMagentoRootDir($start_directory)
    {
        $ds = DIRECTORY_SEPARATOR;
        $directory_tree = explode($ds, $start_directory);

        while (count($directory_tree) > 0) {
            $current_directory = join($ds, $directory_tree);
            $current_search_location = join($ds, array_merge($directory_tree, array('app', 'Mage.php')));

            if (file_exists($current_search_location)) {
                return $current_directory;
            }

            array_pop($directory_tree);
        }

        throw new \Exception("Unable to locate Magento root");
    }