I upgraded to Debian 9 and installed LuaSandbox from the provided package for it to work with PHP 7.0. So far so good. Everything is fluffy and working fine. This is great and thanks a lot for this improvement!
Now I came up with the idea to kill the LuaSandbox stuff I build from source for PHP 5.6. Yeah, I should have done this before. Is it safe to just remove everything that I added to /usr/local/bin/luasandbox and subdirectories as well as /etc/php5/conf.d/luasandbox.ini and /usr/lib/php5/20131226/luasandbox.so? I am not sure if this is somehow mixing with php-luasandbox I installed from the package. I do not think so but one never knows.