After an upgrade to Ubuntu 16.04 a number of sites stopped working due to incompatibility with PHP 7. After a number of attempts to install both PHP 5.6 and PHP 7.0 I succeeded.
By default, sites are running PHP 7.0. In order to switch specific domains back to 5.6, I added the following line in the file
/etc/apache2/sites-available/[my.client.domain].conf
The way I understand, this file will be overwritten the next time the configuration files are rebuilt.
How do I make changes to the domain so that they will survive being rebuilt.