Description:
------------
According to PHP Manual, DateTime class and strtotime functions use relative date and time formats.

http://php.net/manual/en/datetime.formats.relative.php

But using certain relative format in both classes, the result are not the same.

Only PHP 5.6 generates the problem. With PHP >=7.0, the outputs are the same. 

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

echo date('c') . PHP_EOL; // Current Date and Time
$relative = 'first day of next month midnight -1 second';
echo date('c', (new DateTime($relative))->getTimestamp()) . PHP_EOL;
echo date('c', strtotime($relative)) . PHP_EOL;

?>

Expected result:
----------------
2018-01-19T13:51:58-02:00
2018-01-31T23:59:59-02:00
2018-01-31T23:59:59-02:00

Actual result:
--------------
2018-01-19T13:51:58-02:00
2018-01-01T23:59:59-02:00
2018-01-31T23:59:59-02:00