Currently, Response::send() does some backend/server specific shutdown, with the default case being closing userland output buffers. PHP's output buffering may or may not be On by default.

I recently discovered in the course of debugging my Drupal 9 project that closing output buffers may not be enough to flush the response's content to the client. In my case, output buffering wasn't enabled at all, making Response::closeOutputBuffers() a no-op. In either case, it appears necessary to also call flush(). This is important if the calling script/framework wishes to perform additional work after sending the request but before shutting down.

According to the PHP docs for flush(), this seems to be necessary in addition to closing all active output buffers (if used.)

Flushes the system write buffers of PHP and whatever backend PHP is using (CGI, a web server, etc). This attempts to push current output all the way to the browser with a few caveats.

flush() may not be able to override the buffering scheme of your web server and it has no effect on any client-side buffering in the browser. It also doesn't affect PHP's userspace output buffering mechanism. This means ob_flush() should be called before flush() to flush the output buffers if they are in use.

If it is Symfony's intent that the "final" flush() call be done by the implementing code/extending framework, it would be good to explicitly note that.