This quick start guide is geared towards installing PHPUnit on OSX in order to run the WordPress unit tests. It uses homebrew to install PHP using homebrew-php. You can likely skip this step if you've already got php and pear installed properly.
If you use MAMP, then try these instructions to use MAMP's php and pear to install PHPUnit.
ruby <(curl -fsSkL raw.github.com/mxcl/homebrew/go)