If you really want to take serious approach to building web apps using PHP, I would suggest getting on board with Laravel. And signing up with Laracast.com. $9/month is a joke compared to education you’ll get. Things covered are specific to Laravel but Jeffrey Way goes beyond that and covers how to do APIs, how to build robust applications, Object Oriented Programming, and just amazing collection of various interrelated things.
I’ve never used any other PHP framework so take my advice as such. But from my research, CI is falling behind, where Laravel is really picking up steam.