Ubuntu 16.04 comes with PHP7 as the standard, so there are no PHP5 packages
However if you like you can add a PPA to get those packages anyways:
Remove all the stock php packages
List installed php packages with dpkg -l | grep php| awk '{print $2}' |tr "
" " "
then remove unneeded packages with sudo aptitude purge your_packages_here
or if you want to directly remove them all use :
sudo aptitude purge `dpkg -l | grep php| awk '{print $2}' |tr "
" " "`
Add the PPA
sudo add-apt-repository ppa:ondrej/php
Install your PHP Version
sudo apt-get update
sudo apt-get install php5.6
You can install php5.6 modules too ..
Verify your version
sudo php -v