Earlier quoted context omitted.
How bad will be running the built-in web server on public network?
You meant PHP built-in server (php -S)? It doesn't even support concurrent connection. Neither mod_php nor php-fpm is related to the built-in server though. Both are designed for production.
> Set the PHP_CLI_SERVER_WORKERS environment variable to the number of desired workers before starting the server
https://www.php.net/manual/en/features.commandline.webserver...