PHP4 & AUTH procedure

I am migratting to PHP4 from PHP3, and i want to try a basic auth scheme PHP
file example like this:

<%
  if(!isset($PHP_AUTH_USER))
    {
     Header("WWW-authenticate: Basic realm=\"Acceso restringido\"");
     Header("HTTP/1.1 401 Unauthorized");
     echo "To access you need a pair (name,password), SORRY!!";
     exit;
    }
  else
    {
     echo "ALL OK";
     exit;
    }
%>

But that example don't work. In fact, explorer reintent auth dialog 3 times
ever, and fail. (isset($PHP_AUTH_USER) return false ever)

Problem can be from Explorer (5.50), Apache (1.3.14-3) or PHP (4).

(( PHP module was rebuild from sources to support Interbase access, so it
would be a error of me!! ))

Can anybody help me?