How to capture URL that points to non-existent page/file?
Lets say users enters a url like www.myserver.com/randomString, I would like to redirect the page to index.php, but also capture the "randomString"? I'm aware .htaccess can do the redirect on file not found, but what abotu the URL capture?
Use $_SERVER['HTTP_REFERER'] to find out previous address. It cannot be fully trusted since it can be manipulated by some agents.
You may use router-like, let's say, index.php on DocumentRoot and hide it from URL. To elaborate, all the requests will pass from index.php. In your example, it will become www.myserver.com/index.php/randomString and with .htaccess you may hide index.php which will eventually become www.myserver.com/randomString.
Then you may capture whatever you want just analyzing index.php
If it is just for tracking you can use Google Webmaster Tools