The ReadConcern tests (e.g. debug output, serialization) currently test all of the constants, even though ReadConcern itself is not limited to using those constants. Its constructor accepts any string as a level. These tests should be stripped down to just test a single level string. Only readconcern-constants.phpt need exhaustively test each constant value.

This came up in mongodb/mongo-php-driver#1243 for PHPC-1761 when a new constant was being added.