Their GraphQL server is written in PHP. They've made the reference implementation (graphql-js) in JS + the server in Express but they're not using that in produ

Their GraphQL server is written in PHP. They've made the reference implementation (graphql-js) in JS + the server in Express but they're not using that in production. For server side rendering of the React app, they use https://github.com/reactjs/react-php-v8js https://github.com/reactjs/react-php-v8js (well they probably use a slightly customized version but this is what they open sourced).