PHP - Basic Auth - Cpanel

Hi, A client is selling hosting from his site, and _his_ clients get their own control panel from CPanel, for the sake of understanding I will name my client A and his client(s) B. When B needs to log into his cpanel he basicially goes to his a url like this: http://hisSite.com/cpanel/ then the Basic Auth pop-up screen comes up, he enters his user/pass.... My client (A) has got a new damn template which has a "username" and "password" field in it, he wants me to make it so that whenever B enters his username and pass he gets logged into cpanel. My questions: How do I know where to send the data? (eg: which is the authenticating file?) Do I pass it as a GET or a POST or what? URL encode? Thanks, -Ryan A