Unlike any other PHP testing framework that tests applications inside out. Behat is testing applications outside in. It means, that Behat works only with your application's input/output. There are Symfony plugin and Symfony2 bundle for Behat, so you can start testing your applications right now.
Behat is an open source behavior-driven development framework that's PHP 5.3 only inspired by Ruby's Cucumber project. You can install it in one of two ways - either from their custom PEAR channel (pear.everzet.com) or by cloning their github repository. The Behat site has some example tests to get you started.