Description:
------------
If you compute the difference between two DateTimeImmutable objects that differ only by microseconds the difference is correctly calculated (visible by var_dump) but the property value is 0.0.

This problem seems to have begun in 7.2.0RC1, betas are not affected.

https://3v4l.org/sM32U

Test script:
---------------
$dt1 = new \DateTimeImmutable('2017-01-01T00:00:00.000000Z');
$dt2 = new \DateTimeImmutable('2017-01-01T00:00:00.123456Z');
$diff = $dt2->diff($dt1);
//var_dump($diff);
var_dump($diff->f);
var_dump(get_object_vars($diff)['f']);
var_dump($diff->f === get_object_vars($diff)['f']);


Expected result:
----------------
float(0.123456)
float(0.123456)
bool(true)

Actual result:
--------------
float(0)
float(0.123456)
bool(false)