Recently I have been working on a new extension. It is a wrapper around libui, which is a cross platform user interface development library, that allows the creation of native look and feel interfaces in the environments it supports.
That's a few hundred lines of PHP 7 code, moulded into an imitation of the snake game we all used to have on our phones. We've seen other user interface extensions before in PHP, there's even a modified PHP runtime that will allow you to write GTK+ applications.
I don't know anyone that ever deployed any of those extensions, and for very good reasons; PHP5 can barely do anything without allocating a bunch of memory, and doing a bunch of other extremely inefficient things, almost everything it does is inefficient. Beyond a basic forms like application, PHP5 is close to useless.
You can see an example of the snake game in action in this YouTube video. He goes on to talk about the low amounts of CPU and RAM the game (and extension) use and that, with the right amount of work, it can achieve around 60 frames per second. He points out that it is still early on in the development cycle for the extension and libui but there's already documentation for those wanting to investigate.