Behat is an open source Behavior Driven Development framework for PHP that allows testing PHP applications using human-readable sentences to write features and scenarios about how applications should behave. It is a functional testing system that covers scenarios including failures and boundaries. Behavior Driven Development breaks development into defining business value, prioritizing features, breaking features into user stories, and writing code for how features should behave. Functional testing ensures all features and functionality of a system are tested, including failure paths and boundaries. Behat is popular for testing various system types using a readable domain-specific language to write tests in plain English phrases combined into scenarios.