public function sendMessage($to, $from, $body, $variables = array())
    {
        $uri = 'endpoints/sendMessage';
        try {
            $this->client->getEndpoint()->put($uri, [
                'form_params' => [
                    'to' => $to,
                    'from' => $from,
                    'body' => $body,
                    'variables' => array_map('strval', $variables),
                ]
            ]);
        } catch (RequestException $e) {
            $this->processRequestException($e);
        }
    }