Joeri Verdeyen, a developer at Yappa, explains how you can use some Symfony components in a legacy application.

Symfony Components are a set of decoupled and reusable PHP libraries. They are becoming the standard foundation on which the best PHP applications are built. You can use any of these components in any of your applications independently from the Symfony Framework. The purpose of this post is to roughly describe how to implement some of the Symfony Components.

http://tech.yappa.be/symfony-components-in-a-legacy-php-application

Alternatively if you want to use Laravel's Illuminate components, check out Matt Stauffer's Torch repository on GitHub.