How should I structure directories in mvc design pattern in PHP so that I could be able to use the same model and controllers in 'backend' and 'frontend' with different views.
WHAT I HAVE DONE
I am trying to build a website using PHP as server side language and dediced to use MVC design pattern. In my localhost (xampp) I created folder structure like below:
htdocs -- mvc 
           |---- controllers
           |---- models
           |---- views
           |---- library
           |---- system-admin (backend)
               |___ controllers
               |___ models
               |___ views
               |___ index.php (single entry point - backend)
               |___ .htaccess
           |---- index.php (single entry point - frontend)
           |---- .htaccess
I tried to implement basic 'login-system' and created 'Login_Controller' and 'Login_Model' Files.
I GET CONFUSED
How can I be able to use 'Login_Controller' and 'Login_Model' from both  backend and frontend while using seperate views for each. What do I mean is , the url for admins to login will be :
http://localhost/mvc/system-admin/login/ and the url for site users to login will be :
http://localhost/mvc/login/. 
I am designing the backend using twitter-bootstrap framework but the design of frontend will be done by some other who may not use the framework that I have used so I need different views. Both backend and frontend will share the files from library. Some Models and Controllers are also shared (such as Login_Model and Login_Controller) and some are not (such as Dashboard_Model and Dashboard_Contoller will be used only in backend I think) . 
WHAT I NEED
I need some guid on structuring my directories and judgement on my current file structure. I have never used any design pattern (except singleton :-) ) nor any PHP frameworks so I am not able to make any decision on how to structure files.
 
     
    