After updating the OS on my WordPress server to Ubuntu 18.04.1 LTS, I found that PHP was no longer working.

Looking in /etc/apache2/mods-available I could see entries for php7.2 but there was nothing in /etc/apache2/mods-enabled. This seemed fishy.

The fix was simple:

$ a2enmod

Your choices are: ...
Which module(s) do you want to enable (wildcards ok)?

php7.2

 Considering dependency mpm_prefork for php7.2:
 Considering conflict mpm_event for mpm_prefork:
 Considering conflict mpm_worker for mpm_prefork:
 Module mpm_prefork already enabled
 Considering conflict php5 for php7.2:
 Enabling module php7.2.
 To activate the new configuration, you need to run:
 systemctl restart apache2

$ systemctl restart apache2

Et voilà, WordPress was up and running again!