$$unsetOrMatches accepts anything as its argument, and should recursively match an existing value in the actual document against that argument. While POC-ing test changes for DRIVERS-1713, I realized that PHPLIB's Matches constraint doesn't actually handle the case where $exists appears directly within $$unsetOrMatches. This is due to Matches::assertMatchesOperator() not having a case for $exists, which is presently only handled during document iteration.