This library is invaluable to us at Thinkbean as it allows us to integrate into our internal Symfony based devops application which among other things, we use to monitor application status and health.
I run PHP 7 by default on my laptop. For new projects, I typically use Symfony or Drupal 8, and launch the built-in servers with bin/console server:run or drupal server respectively.
For legacy sites that require PHP 5, I have PHP installed in /usr/local/php56/bin and use nginx w/php-fpm to serve them. For these sites, I need to run drush under PHP 5, but PHP 7 is setup as default via the command line.
My solution for this is pretty simple, create an alias named drush5.