When referring to C constants in stubs, PHPC typically uses @cvalue <constant> within a doc block and then defines the actual constant as "UNKNOWN". If we replace "UNKNOWN" with the C constant's defined value, gen_stub.php will generate logic to assert consistency at runtime and we'll be prepared for future improvements that may utilize stubs to generate documentation.

This came out of mongodb/mongo-php-driver/pull#1395 for PHPC-2180.