404 not found!

I am not sure what you mean "alongside"? My main application is in the sub_crud directory. So when I type localhost at the browser, I guess it goes to sub_crud and finds index.php. I guess that index.php is pointing at my controller which is called Configure. In that same controller directory I have another controller called subit_backend.php. This is the one I am trying to get to run from my chrome extension by calling

xhr.open("GET", "http://localhost/subit_backend.php", true);

proof that an old dog can learn new tricks