public static function extract($line)
    {
        $e = explode(': ', $line, 2);
        $header = strtolower(trim($e[0]));
        $value = isset($e[1]) ? trim($e[1]) : null;
        $safe = false;

        foreach (self::$safeCaseValues as $item) {
            if (strncasecmp($header, $item, mb_orig_strlen($item)) === 0) {
                $safe = true;
                break;
            }
            if (strncasecmp($value, $item, mb_orig_strlen($item)) === 0) {
                $safe = true;
                break;
            }
        }

        if (!$safe) {
            $value = strtolower($value);
        }

        return [$header, $value];
    }