Description:
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when working on a date parsed from calendar week from the pattern {YYYY}W{WW}, e.g. "2019W16" strtotime returns the date of Monday in the calendar week 16 in the year 2019, which is 2019-04-15.

When this is combinated with a "next <weekday>" e.g. "2019W16 next friday", the result is one off, in this case 2019-04-18, which is Thursday of that week, not Friday.

Giving "2019-04-15 next friday" gives the correct date: 2019-04-19.

According to my test script, this is broken since PHP 5.2, while working fine in 5.1: https://3v4l.org/0s4Ns

Test script:
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<?php

// see also https://3v4l.org/0s4Ns

if (function_exists('date_default_timezone_set')) date_default_timezone_set("UTC");
echo "works as expected:\n";
echo date("Y-m-d H:i:s", strtotime("2019W14")) . " should be 2019-04-01\n";
echo date("Y-m-d H:i:s", strtotime("2019W15")) . " should be 2019-04-08\n";
echo date("Y-m-d H:i:s", strtotime("2019W16")) . " should be 2019-04-15\n\n";

echo "off by one:\n";
echo date("Y-m-d H:i:s", strtotime("2019W16 next tuesday")) . " should be 2019-04-16\n";
echo date("Y-m-d H:i:s", strtotime("2019W16 next wednesday")) . " should be 2019-04-17\n";
echo date("Y-m-d H:i:s", strtotime("2019W16 next thursday")) . " should be 2019-04-18\n";
echo date("Y-m-d H:i:s", strtotime("2019W16 next friday")) . " should be 2019-04-19\n";
echo date("Y-m-d H:i:s", strtotime("2019W16 next saturday")) . " should be 2019-04-20\n";
echo date("Y-m-d H:i:s", strtotime("2019W16 next sunday")) . " should be 2019-04-21\n";
echo date("Y-m-d H:i:s", strtotime("2019W16 next monday")) . " should be 2019-04-22\n\n";

echo "works as expected:\n";
echo date("Y-m-d H:i:s", strtotime("2019-04-15 next monday")) . " should be 2019-04-22\n";
echo date("Y-m-d H:i:s", strtotime("2019-04-15 next friday")) . " should be 2019-04-19\n";
echo date("Y-m-d H:i:s", strtotime("2019-04-15 next thursday")) . " should be 2019-04-18\n";

Expected result:
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see above.

Actual result:
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see above.