Description:
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There is an issue with the strtotime() function.
http://php.net/manual/en/function.strtotime.php

The expression 'first day of this week' neither returns the expected date nor the "false" value.
It will return different dates depending on the PHP version.

Sometimes, it's the "first day of this month", or the first day of the previous month.


Test script:
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echo date('Y-m-d H:i:s', strtotime('random invalid expression'))."\r\n";
echo date('Y-m-d H:i:s', strtotime('first day of this month'))."\r\n";
echo date('Y-m-d H:i:s', strtotime('first day of this week'))."\r\n";

Expected result:
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1970-01-01 01:00:00
2016-03-01 14:38:03
2016-03-07 14:38:03 // or 1970-01-01 01:00:00

Actual result:
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1970-01-01 01:00:00
2016-03-01 14:38:03
2016-03-01 14:38:03