public function getUserAgent() : string
    {
        $userAgent = '';

        foreach ($this->userAgentHeaders as $header) {
            if (array_key_exists($header, $this->source)
                && $this->source[$header]
            ) {
                $userAgent = $this->cleanParam($this->source[$header]);

                break;
            }
        }

        return $userAgent;
    }