yii\filters\ContentNegotiator::negotiateContentType PHP Method

    protected function negotiateContentType($request, $response)
    {
        if (!empty($this->formatParam) && ($format = $request->get($this->formatParam)) !== null) {
            if (in_array($format, $this->formats)) {
                $response->format = $format;
                $response->acceptMimeType = null;
                $response->acceptParams = [];
                return;
            } else {
                throw new UnsupportedMediaTypeHttpException('The requested response format is not supported: ' . $format);
            }
        }
        $types = $request->getAcceptableContentTypes();
        if (empty($types)) {
            $types['*/*'] = [];
        }
        foreach ($types as $type => $params) {
            if (isset($this->formats[$type])) {
                $response->format = $this->formats[$type];
                $response->acceptMimeType = $type;
                $response->acceptParams = $params;
                return;
            }
        }
        if (isset($types['*/*'])) {
            // return the first format
            foreach ($this->formats as $type => $format) {
                $response->format = $this->formats[$type];
                $response->acceptMimeType = $type;
                $response->acceptParams = [];
                return;
            }
        }
        throw new UnsupportedMediaTypeHttpException('None of your requested content types is supported.');
    }