On the VG Tech blog today there's another post related to unit testing (here's one from before) but this time they're talking about mocking the filesystem with vfsStream, a powerful tool that lets you interact with PHP streams as a virtual file system.
This article is about how to mock the file system when writing unit tests, and it will be rather code-heavy. [...] PHPUnit is the de-facto standard for unit testing in PHP projects, and this is what we will be using together with vfsStream in this article.
The include the code for a simple storage driver (VGF_Storage_Driver_Filesystem) to use with vfsStream with "store", "delete" and "get" methods. Also included are examples of using vfsStream to check things like directory existence, if a file exists, or if a file can be read. A few simple assertions are set up in their sample test to check the methods in their "VGF_Storage_Driver_Filesystem" class.