Ubuntu 16.04 Php5 Apache2
As per http://askubuntu.com/questions/756879/cant-install-php5-on-ubuntu-16-04
Add Ordnj's repository
Also check his website^ more more up to date information.
sudo add-apt-repository ppa:ondrej/php
sudo apt-get update
Install php5 alongside php7
sudo apt-get install php7.0 php5.6 php5.6-mysql php-gettext php5.6-mbstring php-xdebug libapache2-mod-php5.6 libapache2-mod-php7.0
Enable the desired php version
Switch from php5.6 to php7.0
sudo a2dismod php5.6 ; sudo a2enmod php7.0 ; sudo service apache2 restart
Switch from php7.0 to php5.6
sudo a2dismod php7.0 ; sudo a2enmod php5.6 ; sudo service apache2 restart
Checkout oerdnj's github guide.