How can I use $ _SESSION ["REQUEST_URI"] to take the path of the server that I have mounted in PHP?

Goodnight.

I want to use the variable $_SERVER ["REQUEST_URI"] to get the url of my mounted server, but I get the 404 error:

For example:

www.example.com/dir/

I need that path to take it as the index.php and from there I take the routes I need, but for this error I can not get the values ​​I require.

Regards.