yii\httpclient\CurlTransport::prepare PHP Method

    private function prepare($request)
    {
        $request->prepare();
        $curlOptions = $this->composeCurlOptions($request->getOptions());
        $method = strtoupper($request->getMethod());
        switch ($method) {
            case 'POST':
                $curlOptions[CURLOPT_POST] = true;
                break;
            default:
                $curlOptions[CURLOPT_CUSTOMREQUEST] = $method;
        }
        $content = $request->getContent();
        if ($content === null) {
            if ($method === 'HEAD') {
                $curlOptions[CURLOPT_NOBODY] = true;
            }
        } else {
            $curlOptions[CURLOPT_POSTFIELDS] = $content;
        }
        $curlOptions[CURLOPT_RETURNTRANSFER] = true;
        $curlOptions[CURLOPT_URL] = $request->getFullUrl();
        $curlOptions[CURLOPT_HTTPHEADER] = $request->composeHeaderLines();
        return $curlOptions;
    }