Description:
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I think this is just a small bug. 

"W" use ISO-8601 standard and this is enable the following: W01 can start at last year. (Ex. 2020W01 = 2019-12-30 - 2020-01-05)

In this case when I want to get week number - with year information - based on the first day of the week, I get wrong year information.

I tested this with PHP 7.4.0. Everything comes from php.net without any special modules or extensions.

You can reproduce the issue with the "Test script".

Test script:
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$date_string = '2020W01';

$from = date('Y-m-d', strtotime($date_string));
$to = date('Y-m-d', strtotime($date_string . '7'));

echo $from .' - '. $to .'<br/>';

$from = date('Y-W', strtotime($date_string));
$to = date('Y-W', strtotime($date_string .'7'));

echo $from .' - '. $to .'<br/>';

$from = date('Y-W', mktime(0,0,0,12,30,2019));

echo $from;

Expected result:
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2019-12-30 - 2020-01-05
2020-01 - 2020-01
2020-01

Actual result:
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2019-12-30 - 2020-01-05
2019-01 - 2020-01
2019-01