News #6:
It’s one thing to switch from one backend to another in a series of partial and pure tests. It’s quite another to do so in a full-fledged product with numerous dependencies and effects (Aff, S3, Postgres, RabbitMQ, HTTP, etc.). That’s now been done and proven. 
The POC has been updated. Example of async PHP FFI here.
N.b. For HTTP (API), there’s no need for FrankenPHP or anything like it to handle all requests.