You could use smbclient to try to access a known NT service using a user-supplied username and password:
<?php
$servicename = "//server/share";
$username = "pduser";
$password = "userpass";
exec("/usr/bin/smbclient '$servicename' '$password' -U '$username' -c exit", $dummy, $loginfail);
if ($loginfail)
echo "Login failed";
?>
If the user is privileged to access the specified servicename, the login should work. Otherwise it should fail with $loginfail<>0.
Note that if the user is supplying a password to a Web page not via SSL, you've got unencrypted passwords floating around -- a packet sniffer's heaven. It's best to think through the security implications before doing anything like this.
