Behat is an open source Behavior-Driven Development framework for PHP. It is a tool to support you in delivering software that matters through continous communication, deliberate discovery and test-automation.
Behat is like no other testing framework you have used. Instead of proving that your system was built right, it helps you to build the right system in the first place by facilitating and enriching requirements communication.
Behat was built from the ground up for PHP and it is a great PHP library to the core. The codebase heavily uses Symfony2 components, religiously follows coding standards and scores high ratings in major static analysis tools. This all basically means that as a PHP developer, with Behat you will always feel like home.
Behat is very extensible. Almost every bit of the framework functionality could be enhanced or even replaced through the powerful extension system. There are plenty of extensions already available.