While skimming through the hacklang docs today, I noticed Async functionality in there, maybe worth a look?
https://docs.hhvm.com/hack/async/introduction
ZeroMQ seems to have socket support in PHP as well: secure.php.net/manual/en/class.zmqsocket.php
Creating a full socket driven framework might be an interesting task since PHP scripts are typically designed to terminate after they've done their job. Not sure if HHVM would solve that.