http://php.net/mysqli.poll
With Hack's async support, the runtime manages all of this for you. You call "await $query", and then your function is suspended until the data is available. Other code automatically runs until it too hits an await statement -- which might block it on MySQL, memcache, curl, or anything else, with no extra manual coordination needed. The runtime manages it all for you.
http://hhvm.com/blog/7091/async-cooperative-multitasking-for... has some examples for what this looks like with curl -- in the final example, notice how you can just "await" on a bunch of different things and let the runtime worry about coordinating all of them.
The async stuff in HHVM is far more powerful than the meagre-at-best support for asynchronous operations in the original PHP runtime. In usage, HHVM's will be much more powerful.