redirect 404 to similar urls

I have a website with stories in it. I can have multiple types of stories within multiple categories like:

Custom 404 page - PHP

I have a custom 404 page which works fine except for the message I want to display on this page.

How to display Apache"s default 404 page in PHP

I have a webapp that needs to process the URI to find if a page exists in a database. I have no problem directing the URI to the app with .htaccess:

How do I select only the root domain from $_SERVER["HTTP_REFERER"];?

I want to have a 404 page that can detect if a user has come to that page either via my site, via my shortened URL or via another site, and I am making it using PHP. I am slowly getting to grips with PHP and this may well be a simple question, but I am quite tired to be honest and have no caffeine in my system, and I am wanting to tie up any loose ends in my portfolio as soon as possible.