private static function getSignatures($header, $scheme)
    {
        $signatures = [];
        $items = explode(",", $header);

        foreach ($items as $item) {
            $itemParts = explode("=", $item, 2);
            if ($itemParts[0] == $scheme) {
                array_push($signatures, $itemParts[1]);
            }
        }

        return $signatures;
    }