protected static function getWebServiceURL($server)
    {
        $urls = [
            static::USE_PRODUCTION_SERVER  => 'https://api.authorize.net/xml/v1/request.api',
            static::USE_DEVELOPMENT_SERVER => 'https://apitest.authorize.net/xml/v1/request.api',
            static::USE_AKAMAI_SERVER      => 'https://api2.authorize.net/xml/v1/request.api'
        ];
        if (array_key_exists($server, $urls)) {
            return $urls[$server];
        }
        throw new AuthnetInvalidServerException('You did not provide a valid server.');
    }