On the Symfony Finland blog they've posted a look at Symfony's past, present and future in terms of its structure and goals of stability and flexibility. This also includes some of the origins of PHP itself and how it evolved to the stage where creating framework made sense.
I like to think of modern PHP frameworks as glue to put together components to form something that is more than the sum of it's parts. [...] The Symfony Framework is a standard way (and framework code) to create applications using components. The application is always built with a specific structure, which allows code reuse of complete functionalities (Bundles in Symfony lingo) across projects. If you build using a collection of components, you'll need to invest time in learning how that software has decided to use the available components.
He talks more about the idea of components and how they make up a greater whole (like Symfony) and how they relate to the idea of "bundles". He then looks forward to the future of the framework, its long-term support and its work towards being fully PHP7 compatible.
The combination of the PHP language at 20 years and the Symfony framework at 10 years offers a stable platform with flexibility to adapt and grow in the future.