Description:
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As documentation says (http://php.net/manual/en/datetime.createfromformat.php):

If format does not contain the character ! then portions of the generated time which are not specified in format will be set to the current system time.

While in php >= 7.3.0 it's not the case anymore. Date fractions are filled, but time are zeroed.

➜ docker run -v $(pwd):/app --rm php:7.2-cli php /app/dt.php
int(1551845370)
string(29) "2019-03-06T04:09:30.000+00:00"

➜ docker run -v $(pwd):/app --rm php:7.3-cli php /app/dt.php
int(1551845374)
string(29) "2019-03-06T00:00:00.000+00:00"

Test script:
---------------
<?php

$now = \DateTime::createFromFormat('u', 0);

var_dump(time(), $now->format(\DateTime::RFC3339_EXTENDED));