public function current(): string
    {
        if (($ip = &$this->cacheKey(__FUNCTION__)) !== null) {
            return $ip; // Already cached this.
        }
        if ($this->c::isCli()) {
            throw $this->c::issue('Not possible in CLI mode.');
        } // @TODO Try detecting IP address in CLI mode.

        $sources = [
            'HTTP_CF_CONNECTING_IP',
            'HTTP_CLIENT_IP',
            'HTTP_X_FORWARDED_FOR',
            'HTTP_X_FORWARDED',
            'HTTP_X_CLUSTER_CLIENT_IP',
            'HTTP_FORWARDED_FOR',
            'HTTP_FORWARDED',
            'HTTP_VIA',
            'REMOTE_ADDR',
        ];
        foreach ($sources as $_source) {
            if (!empty($_SERVER[$_source]) && is_string($_SERVER[$_source])) {
                if (($_valid_public_ip = $this->getValidPublicFrom($_SERVER[$_source]))) {
                    return $ip = $_valid_public_ip; // IPv4 or IPv6 address.
                }
            } // unset($_valid_public_ip); // Housekeeping.
        } // unset($_source); // Housekeeping.

        if (!empty($_SERVER['REMOTE_ADDR'])) {
            return $ip = mb_strtolower((string) $_SERVER['REMOTE_ADDR']);
        }
        return $ip = 'unknown'; // Not possible.
    }