Ubuntu 16.04 comes with PHP 7.0, and some php applications might still fail with PHP 7.0.

Is not required to remove PHP 7.0. We can install PHP 5.6 together, also because you will have dependency problem with phpMyAdmin package that required PHP 7.0.

To install PHP 5.6 open the terminal and type these commands:

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

Switch PHP version:

From PHP 7.0 to PHP 5.6:

Apache:

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

CLI:

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

From PHP 5.6 to PHP 7.0:

Apache:

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

CLI:

update-alternatives --set php /usr/bin/php7.0

Open the terminal and check your PHP version by running:
php -v