API
From WHMCS Documentation
The API is an interface to allow you to access and perform actions inside WHMCS from external applications and scripts. It allows you to furthur automate and integrate your WHMCS system with other processes you have.
The API is being continually developed and added to as features are requested. If you have a need for a feature that is not currently available, feel free to contact us and let us know and we'll see what we can do.
When adding an API admin user, the user only needs access to the API. You must also setup the IP(s) you will be connecting from in General Settings > Security. Failure to do this will result in an Invalid IP error.