Running Hiphop gave a symbol not found error in the postgres.so file ( undefined symbol: _ZTIN4HPHP11PDOResourceE\n ) exactly like the issue reported on the driver repository (here).
I tried to recompile the postgres driver against Hiphop 3.6.0 but hit a number of problems, mostly to do with hhvm-pgsql-master/pdo_pgsql_statement.cpp it seems.
The fix for the incompatibility was unfortunately rolling back to my previous version of Hiphop. To do this on Mint/Ubuntu just do this:
- Run cat /etc/*-release to get your release information
- Download the appropriate package for your distro from http://dl.hhvm.com/ubuntu/pool/main/h/hhvm/
- Remove your 3.6.0 installation of hhvm: sudo apt-get remove hhvm
- Install the package you downloaded : sudo dpkg -i <deb package>
After that everything should be installed properly and you can start up hhvm without a problem.