Remote UserName

Hmm - an odd one here. I am working in an environment where all browsers are forced to authenticate to a Netscape proxy server and all traffic internal and external is routed through the proxy. Since all users are authenticated as soon as their browser opens, I'd like to be able to access the userid that they used to authenticate in order to handle site personalization, but I can't seem to find any way to get at this information. Any every tried this with php? Cheers, Erik Ableson