I wonder what this means for wikipedia? Will they be migrating to a hack only stack now?
Also, I run a pretty big MediaWiki site and the last time I tried moving everything to HHVM (~6 months ago, I think) stuff appeared to be working initially but randomly broke from time to time (probably because of Scribunto or some other extension; couldn't manage to pinpoint the exact reason). PHP 7 didn't give me any problem at all; it's fast enough (way faster than PHP 5) and most pages are served by the Redis cache anyway. I suspect that Wikipedia will consider a move back to PHP, they're moving the most expensive logic to Lua modules anyway...