On a fresh VPS with debian 8.6 and Webmin I installed PHP7 using the following instructions: https://computerz.solutions/debian-8-installer-php7/ (sorry it's in french but I guess evrybody will be able to read instructions ...)
I then added some lines in Virtualmin -> System settings -> Virtualmin Configuration -> PHP options -> /etc/php /etc/php/7.0 /etc/php/7.0/apache2 /etc/php/7.0/fpm /etc/php/7.0/cli
I also choosed to execute as fast-cgi.
When I create a virtual server I can choose between PHP 5.6 and PHP 7.0 (Virtualmin -> Server configuration -> PHP Versions) so eveything seems to be fine. However when reaching a file with only phpinfo(), I get a 500 error when choosing php 7.0 ... however it works with PHP 5.6 ...
I read somewhere that I should eventually add symbolic links to PHP7 ... not sure about that as Virtualmin seems to see PHP7.
Any idea of where I should look ? Pierre.