There are a few approaches, usually via a standalone node.js service. I've seen similar architectures for ruby and python web servers as well.

I'm working on a project now that uses airbnb's hypernova service[0] with a php client[1].

Edit: not suggesting this was used in this case, just that it's a viable option for react ssr + php.

[0] - https://github.com/airbnb/hypernova

[1] - https://github.com/wayfair/hypernova-php