https://speakerdeck.com/hollodotme/async-php-requests-and-reactive-responses-with-php-fpm "Async PHP Requests & Reactive Responses with PHP-FPM" is talk by Holger Woltersdorf, in which he shares the approaches he tried for implementing asynchronous requests in PHP, and how he arrived at hollodotme/fast-cgi-client, which is a PHP fast CGI client for sending requests (a)synchronously to PHP-FPM.