Web developers know that targeting mobile devices can be tricky. Not only do you have to make sure that your content can handle various screen sizes, you also need to take into account the device orientation. Or you could just figure out an elegant way to sidestep the problem!
Raiper34 has published a library for this very situation. It's called Device rotation prompt and it makes it easy for you to display a prompt on the screen whenever the device is not in the orientation you expect it to be. Neat! And since it's written in JavaScript, integrating it with Phaser is a breeze!