What CMS would be best for me?

Hello guys,

I'm currently an IT student, and I'd like to practice a little by myself with a personal project. I would like to develop an cash register machine that would be operable via an android tablet and in order to work with as many skills as possible, I would like to make it first work as a website, so we can use it from the tablet or the computer. And second, I would like to develop an android app that will do the client side of the app, but I'll do it in the future.

I think I could develop all this from scratch in PHP/MySQL (I already did the database structure) but I would like to gain some skills using a CMS. So My question is: Which one should I go for knowing that:

-I already know how to use PHP with PDO,

-I want it to be easily responsive (so there is an adapted version for the tablet), (I heard that wordpress with bootcamp would be nice)

-I know a little of joomla, but am totally ready to learn a new one from the beginning.

So, I'd be glad if you could give me some advice :)

__Thanks a lot, Reddit!__