On the Weebly Engineering blog there's a new post showing you how to combine PHPUnit and vfsStream to mock out file system operations away from the actual file system.

Recently I found myself needing to write tests for a small class that read from a json file. The class needed to read a json file, validate its existence and content, provide a method to inform the user if a certain key exists, and provide a method to retrieve a value for a given key.

[...] Testing this class in isolation can be tricky because it currently has a dependency on the file system. Storing test json files to test this class would work, but is not ideal because it leaves a dependency on the file system in your tests. As with any external resource, there might be intermittent problems with the file system and could result in some flaky tests. This is where vfsStream shines.

The post includes an example class under test that pulls in the JSON files and operates on the contents. To make the testing easier they introduce vfsStream, a wrapper that allows for a virtual "file system" that can be operated on through the usual interfaces. They include an example of its use in a test on the same class making it easier to check the JSON based on a pre-defined value (essentially a mock of the file and its contents).