Switch php versions on commandline ubuntu 16 04

Question

On my Ubuntu 16.04, I've installed PHP 5.6 and PHP 7.1.

With Apache as my web server, I am aware that I:

a2enmod php5.6 #to enable php5
a2enmod php7.1 #to enable php7

Apache notices the change and uses the PHP 5.6 interpreter as predicted when I disable php7.1 in the Apache modules and enable PHP 5.6.

But when I use the command line to run the internal PHP web server:

php -S localhost:8888

Using PHP 7, PHP processes requests. So how can I change the PHP version in the command line between 5.6 and 7.1?