protected function _checkLdapConfig()
    {
        if (!isset($this->_config['bindDN'])) {
            throw new RuntimeException('Config `bindDN` is not set.');
        }
        if (!is_callable($this->_config['bindDN'])) {
            throw new InvalidArgumentException(sprintf(
                'The `bindDN` config is not a callable. Got `%s` instead.',
                gettype($this->_config['bindDN'])
            ));
        }
        if (!isset($this->_config['host'])) {
            throw new RuntimeException('Config `host` is not set.');
        }
    }