We currently export a few C functions with the PHONGO_API macro:

• phongo_get_std_object_handlers
• zval_to_bson
• bson_to_zval_ex
• bson_to_zval
• php_phongo_bson_typemap_to_state
• phongo_exception_from_mongoc_domain
• phongo_exception_from_phongo_domain

Some of these were originally done in anticipation of providing a standalone BSON extension, which is no longer planned. While this may be an ABI break, we've not documented these exported symbols and there should be no other extensions depending on them to exist.