On the Symfony Finland site there's a post about a relatively new way to run PHP applications and how it's "growing up" to become a viable option: PHP-PM.

PHP-PM is a novel way of running PHP applications. Instead of creating an exotic high performance runtime for the PHP language, it takes an alternative route to mechanism of running PHP applications with existing runtimes.

This translates to real performance gains with existing complex applications, not just impressive theoretical benchmark results.

Instead of the usual complete bootstrap that normal PHP process goes through in its lifecycle, PHP-PM runs them as a continuous process, making for a huge boost in overall performance. The project has started gathering more momentum and is being worked on to make it a more credible platform for PHP applications.

From the humble beginnings the PHP-PM now has over 1700 stars on GitHub and a number of developers working on it. Great strides have been done since the early stages with the documentation and ease of use, but most importantly the platform now supports multiple frameworks: Symfony, Zend and Laravel.