I've been having some kind of problem with this too. This is the code I used based on Bens tutorial and Stephans update.
$salted = md5($input['pass']);
$password = ($salted . $config['salt']);
$stmt->bind_param("ss", $input['user'], $password);
It still isn't working. It just says all usernames and passwords are incorrect, even the correct "admin" "admin". When I hard code in the correct details it processes it fine and goes to the members.php page. So it's connecting to the database correctly and the IF statements to check the input to the database are also working correctly. Any help would be hugely appreciated.