SolidCP does everything via SOAP calls (from portal, to enterprise to server)
Technically you can call this an “api” to call the soap calls to Enterprise (which has pretty much all functions and features).
Its how the whmcs plugin work for example.
Unfortunatly we do not have any proper documentation for this currently.
Though checking the enterprise source should show the api calls, the whmcs plugin source is a nice example of calling those in php.