Since PHPLIB-342, we create an "implicit from the user's perspective" session when a change stream is constructed without an explicit session. Since we do not specify any options when calling Manager::startSession(), this session defaults causalConsistency to true, which results in a ReadConcern being appended to outgoing commands. Since this session is only used for reading, it makes little sense to specify this option. Furthermore, a truly implicit session would never have causalConsistency enabled.

I'll also note that libmongoc defaults causalConsistency to false when creating an implicit session for use with change streams. See: https://github.com/mongodb/mongo-c-driver/blob/1.11.0/src/libmongoc/src/mongoc/mongoc-change-stream.c#L219