public function setOperatorSymbol($symbol)
    {
        if (!in_array($symbol, $this->validOperators)) {
            throw new LdapQueryException(sprintf(
                'Invalid operator symbol "%s". Valid operator symbols are: %s',
                $symbol,
                implode(', ', $this->validOperators)
            ));
        }

        $this->operatorSymbol = $symbol;
    }