On the Symfony Finland site Jani Tarvainen has posted a tutorial showing you how to create asynchronous controller handling in a Symfony-based application.
He then talks about PHP's typical flow model - synchronous and single-threaded. While it does make it simpler to debug/understand it also limits it and can cause higher processing times. Thanks to some other projects, however, asynchronous development with PHP has become more of a reality. He shows how to use one of these projects, Icicle, and its coroutines functionality to make a Symfony controller that handles calls to a
sayHello method asynchronously, returning messages in a fraction of the normal processing time.