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React has been around for a while - http://reactphp.org/

This is not really concurrency, though, it just makes PHP resemble NodeJS.

I don't know if you can classify it as "good" concurrent programming but there's this https://github.com/krakjoe/pthreads

EDIT "pthreads v3 is restricted to operating in CLI only" - from the project page.

This one does look promising. I didn't know they re-written it for PHP7. But still looks like an early days. Might wait a bit!

If I were you I'd explore a language that goes beyond those low level concurrency constructs because they can be extremely tricky to get right. Clojure and Elixir would be my top choices and the final choice depends on the kind of system you're trying to build.

There's amphp[1], and a community effort to make the different sync frameworks interoperable[2].

1: http://amphp.org/

2: https://github.com/async-interop

Using Redux with React or Vuejs is a good way to go!