Why PHP? because is so simple-as-stupid, and for the most case it is all you need to develop simple web-app. I previous work with Codeigniter, i love it. It's well structured with a nice and complete documentation, multiple configuration files and integrates a nice template build system but comes without a good template framework. It's architecture helps to focus on business logic and code business without lose time on wheel diameter.
Even you need a rough private app or a more sophisticated one to release on the Internet, stylish graphic generate more appeal. Beautiful style means great effort but unfortunately Codeigniter (our base system to work with) does not provide tools to build a nice style fast as you can build business logic side of your app.
A few weeks ago I got in touch with Bootstrap, a css framework built by Twitter. It's wonderful and perfect for building nice ready-to-be-so-sexy template. My first own idea was to integrate Bootstrap in Codeigniter framework but as i expected someone else got this idea.
If your web-app need to be used by smartphones your first choice could be to develop a mobile web-app theme using CSS and to write a simple mobile web-app to publish on iOS/Android market. But mobile web-app are usually too much rough to ensure a good user experience.. getting your API may be a better idea.
Everybody has his own method to write down API but if you want a unified solution like phpRocket you must adapt to Codeigniter architecture and this could be painful but Philip Sturgeon is the man who modified Codeigniter to give us a fast and great way to write down complete RESTful API in less than an eye blink.
Writing and using API full-stack framework of phpRocket help you to interface you Android/iOS app with your previous developed phpRocket application in a great way.