Check what alternatives are there for the php:
$ sudo update-alternatives --list php
/usr/bin/hhvm
/usr/bin/php5
If you don't want to keep hhvm as an option at all, you can do:
$ sudo update-alternatives --remove php /usr/bin/hhvm
update-alternatives: removing manually selected alternative - switching php to auto mode
update-alternatives: using /usr/bin/php5 to provide /usr/bin/php (php) in auto mode
This will revert your system to original state. If you want to keep the hhvm, then just change the active alternative:
$ sudo update-alternatives --set php /usr/bin/php5
update-alternatives: using /usr/bin/php5 to provide /usr/bin/php (php) in manual mode
and that's it.