I can't seem to get this to connect to the database so that I can run my prepared statement. Does anybody have an idea what I've forgotten?

private function check_credentials($plain_username, $password)
    {
    global $dbcon;

    $ac = new ac();
    $ac->dbconnect();
    $userid = $dbcon->prepare('SELECT id FROM users WHERE username = :username AND password = :password LIMIT 1');
    $userid->bindParam(':username', $plain_username);
    $userid->bindParam(':password', $password);
    $userid->execute();

    $id = $userid->fetch();
    Return $id;
    }

EDIT: I changed the SQL query from a SELECT FROM query, to an INSERT INTO query and it worked. WHat the heck is going on?