https://askubuntu.com/questions/761713/how-can-i-downgrade-from-php-7-to-php-5-6-on-ubuntu-16-04

1. Re-Install PHP 5.6

What worked for me was this guide: http://www.lornajane.net/posts/2016/php-7-0-and-5-6-on-ubuntu

Actually is not required to remove php7.0, you can install php5.6 together ( also because you will have dependency problem with phpmyadmin package that required php7.0)

Assuming libapache2-mod-php is a suitable way to enable PHP in Apache for you, you can proceed in this way:

sudo add-apt-repository ppa:ondrej/php
sudo apt-get update
sudo apt-get install php7.0 php5.6 php5.6-mysql php-gettext php5.6-mbstring php-mbstring php7.0-mbstring php-xdebug libapache2-mod-php5.6 libapache2-mod-php7.0
2. Switch PHP version:

From php5.6 to php7.0:

Apache:

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

sudo update-alternatives --set php /usr/bin/php7.0
From php7.0 to php5.6:

Apache:

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

sudo update-alternatives --set php /usr/bin/php5.6