Install php 5.6 on Ubuntu 16.04

By default Ubuntu 16.04 (Xenial) now comes with php 7.0

You can install php 5.6 in parallel and switch between them using the following instructions:

add-apt-repository ppa:ondrej/php
apt update
apt install php5.6 libapache2-mod-php5.6 php5.6-curl php5.6-gd php5.6-mbstring php5.6-mcrypt php5.6-mysql php5.6-xml php5.6-xmlrpc
a2dismod php7.0
a2enmod php5.6
systemctl restart apache2
NOTICE: Not enabling PHP 5.6 FPM by default.
NOTICE: To enable PHP 5.6 FPM in Apache2 do:
NOTICE: a2enmod proxy_fcgi setenvif
NOTICE: a2enconf php5.6-fpm
NOTICE: You are seeing this message because you have apache2 package installed.