See: https://github.com/mongodb/mongo-php-driver/blob/1.14.0/php_phongo.c#L183

Going back to PHPC-879, we manually fetch the class entry for php_phongo_date_immutable_ce in MINIT since DateTimeImmutable was not available in some PHP versions. This should no longer be necessary now that the extension requires PHP 7.1+.

date_ce_immutable is private in php_date.c; however, we can use php_date_get_immutable_ce to access it where needed.