In Ubuntu 16, installing PHP with apt-get install php installs PHP version 7, so to install PHP 5.6, you can add a third-party repository.
If the system has PHP 7 installed, then you can delete all of its components like this:
sudo apt-get purge `dpkg -l | grep php| awk '{print $2}' |tr "\n" " "`
Now add a third-party source:
sudo apt-get install software-properties-common sudo add-apt-repository ppa:ondrej/php
And install from it PHP 5.6:
sudo apt-get update sudo apt-get install php5.6
Similarly, you can install modules, for example:
sudo apt-get install php5.6-mysql php5.6-mcrypt php5.6-mbstring php5.6-xml
To check the installed version of PHP, perform:
sudo php -v
See also my article:
Upgrading PHP Version on Ubuntu 14.04