thttpd with php4.02 using the following script always authenicate fail, any idea?

<?php
   if(!isset($PHP_AUTH_USER)) {
     Header("WWW-Authenticate: Basic realm=\"My Realm\"");
     Header("HTTP/1.0 401 Unauthorized");
     echo "Text to send if user hits Cancel button\n";
     exit;
   } else {
     echo "Hello $PHP_AUTH_USER.<P>";
     echo "You entered $PHP_AUTH_PW as your password.<P>";
   }
 ?>