Nginx URL Fallback setup with internal redirection in reverse-proxy settings

[Ashish k]

Hi
    List Members Gurus

                          I have installed and configured a virtual
host using NGINX in Ubuntu 14.04 Now I want to add couple of
functionalities like--

Set up Nginx config so that any URL address which is not found, goes
to a fallback path with internal redirection (NO change or redirection
in browser URL).

Assuming the following:
Fallback path needed for configuration:
http://myvirtualhost/fallback_directory/fallbackhandler.php
Anything typed after http://localhost when not found, should hit the
fallback path (internal redirection, meaning NO change or redirection
in browser address bar). The
fallback path is given above
(http://app_servers/fallback_directory/fallbackhandler.php) which
needs to be setup in Nginx config.

For example, when i visits www.test.com/not_existing_directory/ and
not_existing_directory doesn't exist, it should hit the fallback path
while still retaining www.test.com/not_existing_directory/ in browser
address bar.

Please point me to NGINX resources and a steps that will be required
so that I can grasp NGINX quickly to do the above task.

Thanks in advance
Ashish