Add test case for GH-9601: DateInterval 1.5s added to DateTimeInterface is rounded down since PHP 8.1.0

Author: derickr

NEWS

@@ -10,6 +10,8 @@ PHP                                                                        NEWS
     different type). (Derick)
   . Fixed bug GH-8964 (DateTime object comparison after applying delta less
     than 1 second). (Derick)
+  . Fixed bug GH-9106: (DateInterval 1.5s added to DateTimeInterface is rounded
+    down since PHP 8.1.0). (Derick)
   . Fixed bug #81263 (Wrong result from DateTimeImmutable::diff). (Derick)
 
 - DBA:

ext/date/tests/bug-gh9106.phpt

@@ -0,0 +1,29 @@
+--TEST--
+Test for bug GH-9601: DateInterval 1.5s added to DateTimeInterface is rounded down since PHP 8.1.0
+--INI--
+date.timezone=UTC
+--FILE--
+<?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());
+?>
+--EXPECT--
+int(1577836800)
+int(1577836801)
+int(1577836803)
+int(1577836804)
+int(1577836806)