Recently I updated @world and got new PHP version 7.4. The same machine runs icingaweb2 which is not compatible with PHP 7.4 yet. (https://github.com/Icinga/icingaweb2/issues/4031)
Simple eselect php set apache2 <num>'' with <num> being the 7.2-version (eselect php list apache2) solves the problem with ease.
I'm not sure how this could be solved automatically, since Apache could run anything requiring any version of PHP. Should this be part of Wiki page? I could create it.
I think all that we can enforce via packaging is that you have a version of php that works with icingaweb2 installed, not that it's active. We are already doing that in the package dependencies so I'm going to close this. If it's not part of the upstream docs (the versions of php supported) then that's where it should be.