Add Customer to Mailchimp

        //Add Customer To Mailchimp
        $add_to_mailchimp = 0;
        foreach($transaction->custom_fields->custom_field as $custom_field) {
            if ((string)$custom_field->custom_field_name == "newsletter_subscribe" && (string)$custom_field->custom_field_value == "1") {
                $add_to_mailchimp = 1;
            }
        }
        if ($add_to_mailchimp) {
            require_once 'MCAPI.class.php';
            $mailchimp_api_key = 'xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx';
            $mailchimp_list_id = "xxxxxxxxxx";
            $api = new MCAPI($mailchimp_api_key);
            $merge_vars = array(
                'FNAME' => $customer_first_name,
                'LNAME' => $customer_last_name,
                'OPTINIP' => (string)$transaction->customer_ip
            );
            $retval = $api->listSubscribe($mailchimp_list_id, $customer_email, $merge_vars, "html", false);
        }