To my great stupefaction, php did not work on the 18.04 version. It did work for phpmyadmin. In looking at the configuration files php7.2.conf and phpmyadmin.conf I noticed that in the latter a directive:
AddType application/x-httpd-php .php
existed which was not in the former. Further investigation lead to the discovery that in /etc/mime.types the line(s) for the php extension were marked as comment! Activating at least the line for .php made php working!!