¿Que version de PHP tienes? ¿PHP esta corriendo como cgi en tu servidor o como un modulo de Apache?
Cita: PHP is a feature-rich programming language and they even have a simple HTTP Auhtentication included. The authentication is similar to the Apache one explained here
The bad news is that this type of Authorization does not work when your PHP is installed and working as CGI. It works perfectly when PHP is installed as a module though.
http://www.besthostratings.com/artic...h-php-cgi.html