I’ve just updated HHVM/HHVM-Dev to 3.6 and I started having issues connecting to my database.
It appears that things have changed (https://github.com/facebook/hhvm/issues/5002) and you now need to specify a MySQL socket so it can connect to the server properly. To do this I added the following to my HHVM server.ini. You will need to change it to point at your MySQL socket if your server is local.
hhvm.mysql.socket = /tmp/mysql.sock
Hope this helps others.