Quoting slack:

While vendoring bson-atomic.h into PHPC, I noticed that BSON_IF_MSVC is sometimes redefined, and unlike other defs in that header it's not undefined later. It looks like this could possibly conflict with another definition of the macro in bson-compat.h

The redefinition of BSON_IF_MSVC was introduced in: https://github.com/mongodb/mongo-c-driver/pull/1096