Go to https://dev.freeagent.com/ and register an app.
$ export FREEAGENT_APP_ID='YOUR FREEAGENT APP ID'
$ export FREEAGENT_APP_SECRET='YOUR FREEAGENT APP SECRET'
$ php -S localhost:8000
PHP 5.5.20 Development Server started at Tue Jun 9 19:07:36 2015
Listening on http://localhost:8000