I have followed this article as well, but I am using subdomains so it works for me. Sorry I don’t have an example to show you but I would imagine it wouldn’t be that hard to bind the backend app to a subdirectory.
I’m guessing you could setup your app structure like:
frontend/www/index.php
frontend/www/admin/index.php
and then you’d have to modify your Apache rewrite rules to forward requests to admin to your admin/index.php. Maybe you could even skip putting the admin folder as a subdirectory of the frontend/www folder and just use rewrite rules to forward requests appropriately to backend/www/index.php