Description:
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After initializing a new DateTime object using "first day of this ..." `DateTime::setDate()` doesn't correctly set the day anymore.

It will always result 1.

$date1 = new \DateTime('first day of this month');

echo $date1->format('Y-m-d') . "\n"; // correct: 2017-03-01

$date1->setDate(2017, 1, 8);

echo $date1->format('Y-m-d') . "\n"; // wrong: 2017-01-01

I tried several things like "first day of january" and "first day of this month".

https://3v4l.org/I2maQ

Test script:
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<?php

$date1 = new \DateTime('first day of this month');

echo $date1->format('Y-m-d') . "\n";

$date1->setDate(2016, 1, 8);

echo $date1->format('Y-m-d') . "\n";

$date2 = new \DateTime('first day of january');

echo $date2->format('Y-m-d') . "\n";

$date2->setDate(2016, 3, 8);

echo $date2->format('Y-m-d') . "\n";

Expected result:
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2017-03-01
2016-01-08
2017-01-01
2016-03-08

Actual result:
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2017-03-01
2016-01-01
2017-01-01
2016-03-01