This is a PHP version of Python's StringIO, which is also available in languages like Ruby and Go. It's implemented as a StringIO stream wrapper, with a fictional protocol stringio.
The code is not yet fully finished, but the wrapper might still be useful for some simple unit testing when there is a need to simulate read/write behaviour of a file or stream.
In the future versions it will be possible to pre-populate stream with soma data to enable mocking non-empty file.
Since this wrapper only tries to act as a substitute for a simple file, there is a vfsStream stream wrapper, which provides more complex use cases.