Description:
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From manual page: https://php.net/class.intldateformatter
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According to https://www.php.net/manual/en/function.date.php 'Y' is "A full numeric representation of a year, 4 digits" and 'o' is "ISO-8601 week-numbering year. This has the same value as Y, except that if the ISO week number (W) belongs to the previous or next year, that year is used instead. (added in PHP 5.1.0)"

But in IntlDateFormatter "Y" is "Week of Year", shown in http://userguide.icu-project.org/formatparse/datetime. This means that the following format will be interpreted differently

Test script:
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date = 2018-12-31
format = d-m-Y

PHP date_format "31-12-2019"
IntlDate format "31-12-2020"

Expected result:
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Expected result is the same, but I expect a warning on the PHP IntlDateFormatter page: https://www.php.net/manual/en/class.intldateformatter.php