Description:
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Extend date() formats like
  W	ISO-8601 week number of year, weeks starting on Monday
with
  V	week number of year, weeks starting on Sunday

Example:
date("W V", strtotime("2012-12-30")); // 52 53
date("W V", strtotime("2012-12-31")); // 01 53

E.g. MySQL has date_format() formats:
  %V	Week (01..53), where Sunday is the first day of the week; used with %X
  %v	Week (01..53), where Monday is the first day of the week; used with %x