private function getOperator($operator)
    {
        $operators = $this->getOperators();

        $key = array_search(strtolower($operator), $operators);

        if ($key !== false && array_key_exists($key, $operators)) {
            return $operators[$key];
        }

        $operators = implode(', ', $operators);

        $message = "Operator: $operator cannot be used in an LDAP query. Available operators are $operators";

        throw new InvalidQueryOperator($message);
    }