[php-src] Issue #9106: \DateInterval 1.5s added to DateTimeInterface is rounded down since PHP 8.1.0

Issue: https://github.com/php/php-src/issues/9106 Author: norberttech ### Description The following code: ```php <?php $start = new \DateTimeImmutable("2020-01-01 00:00:00 UTC"); $oneAndHalfSec = new \DateInterval("PT1S"); $oneAndHalfSec->f = 0.5; $t1 = $start->add($oneAndHalfSec); $t2 = $t1->add($oneAndHalfSec); $t3 = $t2->add($oneAndHalfSec); $t4 = $t3->add($oneAndHalfSec); var_dump($start->getTimestamp()); var_dump($t1->getTimestamp()); var_dump($t2->getTimestamp()); var_dump($t3->getTimestamp()); var_dump($t4->getTimestamp()); ``` Resulted in this output in PHP 8.1.0 - 8.1.8: ``` int(1577836800) int(1577836801) int(1577836802) int(1577836803) int(1577836804) ``` But I expected this output instead, like in PHP 7.4.0 - 7.4.30, 8.0.1 - 8.0.21: ``` int(1577836800) int(1577836801) int(1577836803) int(1577836804) int(1577836806) ``` ### PHP Version 8.1.8 ### Operating System _No response_