We have a WHMCS installation and a multicraft installation and then the site itself has a login for the user's profile.
Basically what I'm looking to do is have it so if the user has a site account and they enter their login for WHMCS and multicraft basically tying their accounts together, (Real quick, I know how to process the logins for each of these systems individually so that's not my problem here) I would like to be able to have it so if they login to their site account, during the login process it also logs them into the other services in the background. I'm trying to figure out a good way to implement the "background" part of it. WHMCS has an API to validate the login but I've never worked with API's so I'm not sure yet how to exactly do that but I think I can manage that.
WHMCS API (For those interested): http://docs.whmcs.com/API:Validate_Login