Alessandro Nadalin has posted a two part series to his site to help you get the Twig templating tool integrated with your legacy codebase.

It might happen that you are working on a legacy code that is years old, with its own templating mechanism1 that doesn't really allow you to take advantage of the benefits that a structured and object-oriented engine like Twig. In this situations, when a complete replacement would cost too much to your organization, you can take advantage of a wild integration between this advanced template engine and your existing code.

In the first part of the series, he proposes integrating it at the base level, essentially wrapping the "render" method of your framework of choice. In the second part of the series, though, he comes back with a "less wild" approach that also lets you take advantage of some of the other features twig has to offer. He modifies the previous example to output a template file and reuse that in the Twig "render" call.