byte-stream/docs/in-memory-stream.md

InMemoryStream

An InMemoryStream allows creating an InputStream from a single known string chunk. This is helpful if the complete stream contents are already known.

$inputStream = new InMemoryStream("foobar");

It also allows creating a stream without any chunks by passing null as chunk.

$inputStream = new InMemoryStream;

// The stream ends immediately
assert(null === yield $inputStream->read());