public static function getSignatureString(array $signatureHeaders)
    {
        $signature = '';

        foreach ($signatureHeaders as $header => $value) {
            if ($signature !== '') {
                $signature .= "\n";
            }

            if (is_array($value)) {
                $value = $value[0];
            }

            $signature = mb_strtolower($header) === "request-line"
                ? "{$signature}{$value}"
                : $signature . mb_strtolower($header) . ": {$value}";
        }

        return $signature;
    }