A quick new post to the NetBeans blog today points out the support for Selenium in the IDE. The NetBeans wiki has more information:

Before trying Selenium in PHP be sure to have PHPUnit and Selenium_Testing packages installed. The recommended way to install both is using PEAR.

The wiki page guides you through the creation of a simple testing setup complete with screenshots of the interface the whole way through. The plugin page has more details on running the tests.