Phing is a PHP tool that allows you to build projects. Phing stands for PHing Is Not Gnu make, which is kind of a common coder's joke. It is basically a tool that will take a set of files in multiple directories and create either a zip file or a numerous other things that you might want to do with a project.
Phing itself is a PHP program, so all that is needed to run it is a working copy of PHP. Also, because Phing is part of PEAR it is easy to install, and you don't need to install Apache to run it.