On the Stubbles blog, Frank Kleine has posted about the release of a new wrapper class he's developed to go around a virtual file system.

Some minutes ago I released the first version of vfsStream. vfsStream is a stream wrapper for a virtual file system that may be helpful in unit tests to mock the real file system. It can be used with any unit test framework, like PHPUnit or SimpleTest.

The idea for the class came about when he thought about the use of real databases vs test databases and applied it to file systems. You can grab the latest version from their trac website as well as get more information about known issues and a brief example of its use.