Description:
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I use this code to calculate the week number for December 28 of 2009:

strftime("%V", mktime(12,0,0,12,28,$x));

This function have to return "1" but it returns "53" and the 2009 year hasn't 53 weeks.

I also tried other code with the same results:

date('W', strtotime("December 28, $x"));