At my previous place of work, we used the V8js extension to fire up a V8 VM, send data to it and render react components to strings. Worked really well, especially with some caching in place.

Of course, we also moved to Amp and a PHP-cli-running-async-http-server model, so we can share the in memory extension, and did some cute stuff with Redis to get it running stupidly fast without memory leaks. Good fun really, PHP 7.1 is damned nice in a lot of ways, but annoying in others still.