[php-src] Issue #9761: fix $datetime->modify("now")

Issue: https://github.com/php/php-src/issues/9761 Author: divinity76 ### Description currently (and imo, surprisingly) when doing ```php <?php $dt = new DateTime(); sleep(2); $dt->modify("now"); ``` the modify() call does.. absolutely nothing. imo this is bad: if it is truly an unsupported operation, modify() should have thrown, or at least made a warning like $dt->modify("Chuck Norris"); generates Warning: DateTime::modify(): Failed to parse time string (Chuck Norris) at position 0 (C) but it would be even better if it was a supported operation, changing the timestamp to the current time, like doing $dt->setTimestamp(time()); with millisecond/microsecond/etc precision