ArBDD is a simple yet powerful BDD library for PHP to get you started with BDD in PHP. Excellent for those who have no previous experience with BDD.

The project's page includes some basic code definitions of behaviors the code should follow, the creation of a simple mock object and the JSON-based results that come out when the tool is executed. They've also created an AIR-based viewing tool to see the results outside the browser. The code requires PHP 5.3+ because of its use of closures.

For another example of BDD testing in action, look at the Behat framework, based on the Ruby Cucumber syntax.