oAuth

This technique has the advantage that you don't need to store the user credentials in your application, because the complete authentication is handled by OAuth. OAuth is mandatory for applications allowing many users to access the API. To start using OAuth, you should register your application inside MoneyBird. Inside your MoneyBird account, visit the url /oauth_clients/new to register your application and start using OAuth.

See the oAuth website for information about oAuth. How oAuth works in Zend Framework (with good explanation) can be found here.