In a recent post to his site Jeff Geerling shows you how to disable the output buffering that PHP includes and create "streaming PHP" code similar to Drupal's recently introduced BigPipe handling.
For the past few days, I've been diving deep into testing Drupal 8's experimental new BigPipe feature, which allows Drupal page requests for authenticated users to be streamed and loaded in stages—cached elements (usually the majority of a page) are loaded almost immediately, meaning the end user can interact with the main elements on the page very quickly, then other uncacheable elements are loaded in as Drupal is able to render them.
[...] BigPipe takes advantage of streaming PHP responses (using flush() to flush the output buffer at various times during a page load), but to ensure the stream is delivered all the way from PHP through to the client, you need to make sure your entire webserver and proxying stack streams the request directly, with no buffering.
He decided to try out different configurations to see if he could reproduce the same thing outside of Drupal and - good news, everyone - he found a reliable way. He starts with a basic procedural script that emulates BigPipe and calls a flush inside a loop to push the latest output to the waiting client. While this cooperates on the command line the browser doesn't cooperate the same way. A small tweak helps it work, so he shows how to reproduce this reliably across the full stack - Nginx, Apache and Varnish.
He ends with a quick warning for those using VMWare/VirtualBox about some oddness he experienced in buffering the responses and includes a way to test if it's your script or the VM causing the trouble.