In my case it is rather:
$ lcclient --diag
...
- PHP 5.6.40 (code='php56')
CGI/FastCGI: /opt/php-5.6/bin/php-cgi
FPM: /opt/php-5.6/sbin/php-fpm
pool config: /etc/php-fpm/php56-fpm.d
default php.ini: '/opt/php-5.6/etc/php.ini'
- PHP 7.2.24 [DEFAULT] (code='php7')
CGI/FastCGI: /usr/bin/php-cgi
FPM: /usr/sbin/php-fpm7.2
pool config: /etc/php/7.2/fpm/pool.d
default php.ini: '/etc/php/7.2/cgi/php.ini'
- default PHP CLI: /usr/bin/php
...
Then I tried again to get it working with fpm and I only managed after manually restarting services like this:
systemctl stop apache2 ; systemctl stop php7.2-fpm; systemctl stop php56-fpm; systemctl start apache2; systemctl start php7.2-fpm; systemctl start php56-fpm
Apparently some 7.2 services remain running which inhibits 5.6 to take charge of the site, at least that is how it looks like to me.