GuzzleHttp\Client::applyOptions PHP Method

    private function applyOptions(RequestInterface $request, array &$options)
    {
        $modify = [];
        if (isset($options['form_params'])) {
            if (isset($options['multipart'])) {
                throw new \InvalidArgumentException('You cannot use ' . 'form_params and multipart at the same time. Use the ' . 'form_params option if you want to send application/' . 'x-www-form-urlencoded requests, and the multipart ' . 'option to send multipart/form-data requests.');
            }
            $options['body'] = http_build_query($options['form_params'], '', '&');
            unset($options['form_params']);
            $options['_conditional']['Content-Type'] = 'application/x-www-form-urlencoded';
        }
        if (isset($options['multipart'])) {
            $options['body'] = new Psr7\MultipartStream($options['multipart']);
            unset($options['multipart']);
        }
        if (isset($options['json'])) {
            $options['body'] = \GuzzleHttp\json_encode($options['json']);
            unset($options['json']);
            $options['_conditional']['Content-Type'] = 'application/json';
        }
        if (!empty($options['decode_content']) && $options['decode_content'] !== true) {
            $modify['set_headers']['Accept-Encoding'] = $options['decode_content'];
        }
        if (isset($options['headers'])) {
            if (isset($modify['set_headers'])) {
                $modify['set_headers'] = $options['headers'] + $modify['set_headers'];
            } else {
                $modify['set_headers'] = $options['headers'];
            }
            unset($options['headers']);
        }
        if (isset($options['body'])) {
            if (is_array($options['body'])) {
                $this->invalidBody();
            }
            $modify['body'] = Psr7\stream_for($options['body']);
            unset($options['body']);
        }
        if (!empty($options['auth']) && is_array($options['auth'])) {
            $value = $options['auth'];
            $type = isset($value[2]) ? strtolower($value[2]) : 'basic';
            switch ($type) {
                case 'basic':
                    $modify['set_headers']['Authorization'] = 'Basic ' . base64_encode("{$value['0']}:{$value['1']}");
                    break;
                case 'digest':
                    // @todo: Do not rely on curl
                    $options['curl'][CURLOPT_HTTPAUTH] = CURLAUTH_DIGEST;
                    $options['curl'][CURLOPT_USERPWD] = "{$value['0']}:{$value['1']}";
                    break;
            }
        }
        if (isset($options['query'])) {
            $value = $options['query'];
            if (is_array($value)) {
                $value = http_build_query($value, null, '&', PHP_QUERY_RFC3986);
            }
            if (!is_string($value)) {
                throw new \InvalidArgumentException('query must be a string or array');
            }
            $modify['query'] = $value;
            unset($options['query']);
        }
        // Ensure that sink is not an invalid value.
        if (isset($options['sink'])) {
            // TODO: Add more sink validation?
            if (is_bool($options['sink'])) {
                throw new \InvalidArgumentException('sink must not be a boolean');
            }
        }
        $request = Psr7\modify_request($request, $modify);
        if ($request->getBody() instanceof Psr7\MultipartStream) {
            // Use a multipart/form-data POST if a Content-Type is not set.
            $options['_conditional']['Content-Type'] = 'multipart/form-data; boundary=' . $request->getBody()->getBoundary();
        }
        // Merge in conditional headers if they are not present.
        if (isset($options['_conditional'])) {
            // Build up the changes so it's in a single clone of the message.
            $modify = [];
            foreach ($options['_conditional'] as $k => $v) {
                if (!$request->hasHeader($k)) {
                    $modify['set_headers'][$k] = $v;
                }
            }
            $request = Psr7\modify_request($request, $modify);
            // Don't pass this internal value along to middleware/handlers.
            unset($options['_conditional']);
        }
        return $request;
    }