add-apt-repository ppa:ondrej/php
apt-get update
apt-get install php5.6 php5.6-mysql php5.6-mbstring libapache2-mod-php5.6 php5.6-intl php5.6-gd php5.6-xml php5.6-zip php5.6-curl php-apcu

From php5.6 to php7.0:

Apache:

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

CLI:

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

From php7.0 to php5.6:

Apache:

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

CLI:

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