Using the memory/temporary stream provided by php:// stream wrapper you can create a stream with read and write access directly to RAM or to a temporary file [using "php://memory"]. This gives you the possibilty to write unit tests that does not rely on a specific file, resource or stream, but rather on data provided by the test itself.
There's no specific code examples here, but you can refer to the stream wrappers section of the PHP manual for more details on this and other handy built-in streams. Once created, it can then be used just as any other stream resource can. This could be useful to provide mocks in your testing, replacing any other stream-able resource with a "memory" or "temp" placeholder.