Ubuntu 16.04 doesn’t natively let you install PHP5.6. It only lets you install PHP 7.0. It is a long process to install PHP 5.6 if your server is not allowed to connect to the internet.
Drupal 7, as of Drupal 7.44, (approximately July 2016) supports and works with PHP 7.0 .So you may install PHP 7.0 on Ubuntu 16.04 for Drupal 7 sites instead of jumping through the hoops to get PHP5.6 installed on the server.
Reference: Ubuntu Drupal 7 package