NTLM authentication with IIS and latest IE

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I've gotten NTLM authentication working with PHP 5.0.2 and IIS 5.0, so
long as I use Internet Explorer 6.0.2800.  Using IE 6.0.2900
authentication doesn't happen.  IE displays a "Cannot find Server or
DNS Error" page ("friendly HTTP message") or just hangs.  Anybody know a
workaround?  Here's the current incarnation of my code:

   header('HTTP/1.0 401 Authorization Required');
   header('WWW-Authenticate: Negotiate');
   header('WWW-Authenticate: NTLM');
   header('X-Test-Header: Am I seeing double?');
   echo 'This page requires EVNT authentication';

(EVNT refers to the realm I'm trying to authenticate.)

Any ideas what I can do to make NTLM work with all versions of IE?

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