Can I use fastcgi_finish_request() like register_shutdown_function?

Question

This simple method for caching dynamic content uses register_shutdown_function() to push the output buffer to a file on disk after exiting the script. However, I'm using PHP-FPM, with which this doesn't work; a 5-second sleep added to the function indeed causes a 5-second delay in executing the script from the browser. A commenter in the PHP docs notes that there's a special function for PHP-FPM users, namely fastcgi_finish_request(). There's not much documentation for this particular function, however.

The point of fastcgi_finish_request() seems to be to flush all data and proceed with other tasks, but what I want to achieve, as would normally work with register_shutdown_function(), is basically to put the contents of the output buffer into a file without the user having to wait for this to finish.

Is there any way to achieve this under PHP-FPM, with fastcgi_finish_request() or another function?

$timeout = 3600; // cache time-out
$file = '/home/example.com/public_html/cache/' . md5($_SERVER['REQUEST_URI']); // unique id for this page
if (file_exists($file) && (filemtime($file) + $timeout) > time()) {
  readfile($file);
  exit();
} else {
  ob_start();
  register_shutdown_function(function () use ($file) {
    // sleep(5);
    $content = ob_get_flush();
    file_put_contents($file, $content);
  });
}

Answer 1

Yes, it's possible to use fastcgi_finish_request for that. You can save this file and see that it works:

<?php

$timeout = 3600; // cache time-out
$file = '/home/galymzhan/www/ps/' . md5($_SERVER['REQUEST_URI']); // unique id for this page
if (file_exists($file) && (filemtime($file) + $timeout) > time()) {
  echo "Got this from cache<br>";
  readfile($file);
  exit();
} else {
  ob_start();
  echo "Content to be cached<br>";

  $content = ob_get_flush();
  fastcgi_finish_request();
  // sleep(5);

  file_put_contents($file, $content);
}

Even if you uncomment the line with sleep(5), you'll see that page still opens instantly because fastcgi_finish_request sends data back to browser and then proceeds with whatever code is written after it.