At SpinGo, we've been building our software on Scala, a staticly-typed, multi-paradigm programming language for the JVM. We've enjoyed it and would like to share our experience.
Two years ago, the entire backend was written in CakePHP (with the traditional server-side-rendered); we've felt that the rich-client / API approach to writing software can offer far better user experience; additionally, tracking some amount of state in the browser can make many interaction patterns drastically simpler to implement. Our PHP stack had become tremendously difficult to maintain; it lacked Unit tests, the coding style was heavily imperative, and 500+ line methods were less rare than I'd like. Further, PHP did not meet many of our specific list of desirable features in a programming language, namely: