Using the Drip API Wrapper for PHP (https://github.com/DripEmail/drip-php) is super easy. Here is an example.
You can get your Drip (http://www.getdrip.com) API token by logging into Drip and going to your User Settings.
Set up a workflow in Drip so that when the tag "new_user" is applied it starts the onboarding email course that you created.