Any recommended light weigh u-MVC for single app/limited users’ app.

Hi I’m in my way to build web app, that had about 5 users with single admin, I’m trying to build this system the simplest way as I can, BTW the users mostly be fixed which means there is no registration and the admin do not need to create new users,

I was thinking using CodeIgniter even it’s a lightweight framework, but I’m still looking for much simpler one .

=Edit= This Billing system for small office, main goal convert (Paper & Excel bills) to data store in DB, and I can generate reports off it (for period of time e.g. last 3 months). BTW will need sort of API to connected between some of these users, since one of them will installed on other PC (server) .