Whmcs integration With cloud stack

HI
I want to have Whmcs as a billing solution for my cloud stack what i want to know how Whmcs is going to integrate with cloud stack (i.e. calling the Api and fetching the data and stuff) Whmcs integration with the website is fairly easy you just have to change the header and footer files . but what i want to know is how whmcs is going to call the API's of cloud stack and fetch data and show results

Any help in this regard will be great

Thanks :)