Current behavior in php_phongo_bson_visit_binary() derives the hydration class from the last field it encounters that (1) containing binary data (2) with a sub-type of 0x80 and (3) contains a valid class name (after autoloading).

While this is deterministic and clever, since it does not depend on a particular field name, it could lead to confusion among users. Using a dedicated, short field name that implies we're actually storing a PHP or HHVM class name would be more straightforward.