public function subscribe($email_id, $email_type = 'html', $double_optin = true, $update_existing = true, $replace_interests = true, $send_welcome = false, $email_identifier = 'email') {
        if (!in_array($email_identifier, array("email", "euid", "leid")))
            throw new InvalidArgumentException('email identifier should be one of ("email","euid","leid")');

        $payload = array(
            'id' => $this->listId,
            'email' => array(
                $email_identifier => $email_id
            ),
            'merge_vars' => $this->merge_vars,
            'email_type' => $email_type,
            'double_optin' => $double_optin,
            'update_existing' => $update_existing,
            'replace_interests' => $replace_interests,
            'send_welcome' => $send_welcome
        );

        $apiCall = 'lists/subscribe';
        $data = $this->requestMonkey($apiCall, $payload);
        $data = json_decode($data, true);
        if (isset($data['error']))
            throw new MailchimpAPIException($data);
        else
            return isset($data) ? $data : false;
    }