How can I downgrade from PHP 7 to PHP 5.6 on Ubuntu 16.04?

ubuntu-php-version-switch.txt

Switch PHP version:

From php5.6 to php7.0 :

Apache:
sudo a2dismod php5.6 ; sudo a2enmod php7.0 ; sudo service apache2 restart

CLI:
sudo ln -sfn /usr/bin/php7.0 /etc/alternatives/php

From php7.0 to php5.6:

Apache:
sudo a2dismod php7.0 ; sudo a2enmod php5.6 ; sudo service apache2 restart

CLI:
sudo ln -sfn /usr/bin/php5.6 /etc/alternatives/php

if after you follow the below instructions you face this situation, you may need to disable the proxy_fcgi apache module:

sudo a2dismod proxy_fcgi proxy; sudo service apache2 restart