PHP has a built in debugging and testing tool called assertions, which is essentially the assert() function and a few configuration options.
Providing a Phing build file along with a project is a good way of allowing automation of certain aspects of the project. The only trouble is that users won't know what's in the build file unless they open it or just run it.
Phing has a few different tasks and elements that allow you to select paths of code execution depending on what you need to happen in a build file. These are limited to loops and if statements, but a lot of functionality can be covered with just a couple of lines of XML.
The other day I was experimenting with Git hooks. These are scripts that you can execute before certain actions are run in Git. For example, you might want to ensure that forced updates are not run, ensuring respository files have the correct permissions after merging, or that the files have ASCII standard names before being committed.
I use Phing for a lot of different tasks, it helps me to automate things that I would otherwise mess up if left to my own devices. Prime candidates for Phing scripts are things that I don't do that much and forget how to do them, or that have a number of complex steps.