It's an interesting question. I think there might be PHP in 20 years. But not as dominant as today. It will be more like a forgotten language which was used on the old days of the web.
Javascript is trending rapidly right now. And with ES6/Next evolving more and more. Besides that, i think that the web in 20 years will be more about native browser apps then html / php websites.
However I think PHP will be solid for the next few years, it is still very popular.