Description:
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This is actually a nice feature for calender data/time operations, but not documented yet:

Specifying a week number larger than valid for the year, the end results nicely overflows into the next year.



Test script:
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<?php
echo date( ’W’, strtotime( "December 28th, 2011" ) ), "\n";
$ndate = new DateTime();
$ndate->setISODate(2011,56,07);
echo "ndate: " . $ndate->format('Y-m-d') . "\n";
$ndate->setISODate(2012,04,07);
echo "ndate: " . $ndate->format('Y-m-d') . "\n";
?>

Expected result:
----------------
52
ndate: 2012-01-29 
ndate: 2012-01-29 

Actual result:
--------------
52
ndate: 2012-01-29 
ndate: 2012-01-29