Description:
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From manual page: http://www.php.net/function.strtotime
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Since 5.1.0, a date containing a day >= 32 is no longer interpreted as overflowing to the next month. Day = 31 is still accepted for months that have 30 days or less, and is interpreted as day 1 of the next month.

Test script:
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echo date('Y-m-d', strtotime('2016-04-31'));
echo date('Y-m-d', strtotime('2016-05-32'));

First case returns 2016-05-01, in all PHP versions.
Second case returns 2016-06-01, before 5.1.0, and 1970-01-01 after 5.1.0.