mongoc_write_concern_is_valid is called only once in the driver near the end of php_phongo_apply_wc_options_to_uri.

mongoc_read_prefs_is_valid is called from php_phongo_apply_rp_options_to_uri and at the end of the ReadPreference constructor. As discussed in a PR comment for PHPC-1571, calling this at the end of the constructor likely serves as a fail safe in case we missed throwing a more helpful exception for incompatible options.

I'd propose we these is_valid checks from each class' init_from_hash function, as well as introduce a final call to mongoc_write_concern_is_valid in the WriteConcern constructor.