Fabien Potencier has new post post to the Symfony Blog today talking about a big update to the Symfony framework, the introduction of a new "fragment sub-framework". This framework (now a part of the HttpKernel) allows the handling of requests based on a few different strategies.
About a month ago, I merged a complete refactoring of the sub-requests management of Symfony. In fact, I created a whole new sub-framework to handle the rendering of resource fragments via different strategies. [...] Besides classical master requests, the HttpKernel component is now able to handle sub-requests. Let's me sum up the different strategies that are available: internal sub-requests, ESIs, HIncludes, and SSIs (in 2.3).
He talks some about each strategy and what situations they're best suited for and some sample code that shows the use of the "standalone" and "strategy" parameters. There's also new methods you can call (like "render_esi" or "render_hinclude") to work with the new features as well.