Description:
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When using "first day of" relative time formats, the constructor gives inconsistent values for what ought to be the same. Results shown below are all as of 2020-03-22 03:49:52.

Test script:
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$month_start = new DateTime("first day of this month");
echo $month_start->format('Y-m-d H:i:s') . "\n";
$month_start = new DateTime("first day of March");
echo $month_start->format('Y-m-d H:i:s') . "\n";

https://3v4l.org/lG3GL

Expected result:
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2020-03-01 00:00:00
2020-03-01 00:00:00

or possibly

2020-03-01 03:49:52
2020-03-01 03:49:52


Actual result:
--------------
2020-03-01 03:49:52
2020-03-01 00:00:00