I have just found out that in PHP 7.4, the builtin webserver (php -S) supports multiple workers! From https://www.php.net/manual/en/features.commandline.webserver.php
|7.4.0||You can configure the built-in webserver to fork multiple workers in order to test code that requires multiple concurrent requests to the built-in webserver. Set the PHP_CLI_SERVER_WORKERS environment variable to the number of desired workers before starting the server. This is not supported on Windows.|
It defaults to 1.