Cucumber is a Behavioural Driven Development framework written in Ruby. Using a wire it allows to write BDD steps in PHP to test PHP applications. Cuke4php is in fact a wire protocol implementation of Cucumber written in PHP.
You'll still need to have Cucumber (Ruby) installed on your testing system, but pulling in the Cuke4PHP tool is as easy as pointing your "pear" command-line tool at their PEAR server and installing. With a little bit of configuration (and possibly an install of PHPUnit) you'll be up and running. You can then write Cucumber tests that can run against your PHP code. They've included a sample PHP class and a Cucumber test to match it.