note 123447 added to datetime.format

There is a bit confusing logic may appear using year week number: <?php echo (new \DateTime("2018-12-31 13:05:21"))->format("YW") . PHP_EOL; ?> will output 201801, not 201901 nor 201852, because of strange ISO_8601-2004 standard: the first calendar week of a year is that one which includes the first Thursday of that year, so this date (it is Monday) do belong to the first week of 2019 (this is why 'W' format gives 01), but internal timestamp is of 2018 (and 'Y' format obey this), therefore getting us unexpected result of 201801. So be careful when using this output with something important (i know projects where this was used to form MySQL partitions). ---- Server IP: 208.43.231.9 Probable Submitter: 193.201.100.20 ---- Manual Page -- http://php.net/manual/en/datetime.format.php Edit -- https://master.php.net/note/edit/123447 Del: integrated -- https://master.php.net/note/delete/123447/integrated Del: useless -- https://master.php.net/note/delete/123447/useless Del: bad code -- https://master.php.net/note/delete/123447/bad+code Del: spam -- https://master.php.net/note/delete/123447/spam Del: non-english -- https://master.php.net/note/delete/123447/non-english Del: in docs -- https://master.php.net/note/delete/123447/in+docs Del: other reasons-- https://master.php.net/note/delete/123447 Reject -- https://master.php.net/note/reject/123447 Search -- https://master.php.net/manage/user-notes.php