How can I authenticate a user in my vbulletin database from a php script?

Here's a php snippet that connects to your vbulletin database and checks a username / password against it.  This is useful for having other scripts on your site authenticate users against the same registered user list as vbulletin.

<?

$host = "hostname";
$user = "username";
$pass = "password";
$db = "database";

$conn = mysql_connect($host, $user, $pass);
mysql_select_db($db, $conn);
$sql = "SELECT userid, usergroupid, membergroupids, infractiongroupids, username, password, salt FROM vb_user WHERE username = '" . htmlspecialchars($username). "'";

$r = mysql_query($sql,$conn);
$results = mysql_fetch_array($r, MYSQL_ASSOC);

mysql_free_result($r);

if($results['password'] != '' && $results['password'] == md5(md5($password).$results['salt'])) {
 // this is a match
}
else {
 // this is not a match
}
?>