WHMCS User Password Synchronization with PHP/MySQL custom App

I have a custom PHP/ MySQL database and application which maintains user IDs and passwords. I need to create an integration with my WHMCS implementation so that users on my custom database do not need to create separate account on WHMCS. In other words, a user should be able to use the same credentials as my custom portal on WHMCS portal as well. In addition, when the user is logged into my application, they shouldnt have to login again if they visit the WHMCS link. In other words, we should auto authenticate them so they are signed into WHMCS when they land on that page. Finally, we need to ensure there is ongoing synchronization of user information (name, address, password, email address) between the two databases. So if I change my user information in the custom app/database , those changes should be synchronized with WHMCS. You can read about WHMCS APIs here. [login to view URL] You can pay attention to these functions = AddClient, UpdateClient, ValidateLogin Please indicate in your response if you have prior WHMCS experience and/or if you have done similar type of work in the past.