Opening this as a tracking ticket. We can debate whether it is worth implementing down the line.
PHPC-460, our BSON types are serializing the same properties reported by var_dump(), since we use the same get_properties handler. Since both Timestamp components are unsigned 32-bit integers, a 64-bit PHP system might report a positive integer (e.g. 4294967295) that cannot be unserialized on a 32-bit platform.
Since UTCDateTime is a 64-bit integer, we'd likely serialize it as a string on 32-bit platforms and an integer on 64-bit platforms.