On the Symfony Finland site, there's a post that looks at the current state of GraphQL PHP libraries in 2017 and how they are integrated with applications using the Symfony framework.
GraphQL has continued to gain momentum over the course of 2017. While it's certainly not a replacement for REST in all cases, it does provide consumers of Content APIs, etc. better ergonomics than general purpose RESTful interfaces. For PHP there are currently two popular libraries, both of which have Symfony integration Bundles.
The post starts by talking about implementing GraphQL functionality in your own application and mentions the two main libraries currently used: Webonyx GraphQL PHP and Youshido GraphQL. It goes on to talk about the integrations both of these provide as bundles and a bit about what each has to offer. The post then wraps up with a look forward to the Symfony Flex support they provide and a few links to other resources about GraphQL and its use in Symfony applications.