According to this new post on the NetBeans blog (from Sun), native support has been added to the IDE for PHPUnit, a popular PHP-based unit testing utility.

Today I would like to inform you that we have added PHPUnit support to NetBeans. It is not finished yet (UI for test results is still missing) but I would like to show you how one can test his/her PHP sources in NetBeans today. If you are new to PHPUnit, be sure that you visit PHPUnit web site first to get all the required information.

They walk you through the setup of the IDE's support for the tool (once you have PHPUnit already installed) and how to set up your first unit test with it. One handy feature they've included is the ability to link classes with the unit tests that affect them via a popup menu.