Getting the Week of the Year

I'm trying to make a PHP-based site that archives by week, like Fark does. Thing is, I don't know how to query by week of the year.

How do I do that?

>>12
even wikipedia, which is full of eurofags, says:
Its name in Arabic, Armenian, Georgian, Greek, Hebrew, Persian, Portuguese and Syriac means "second day". Quakers also traditionally refer to Monday as "Second Day" eschewing the pagan origin of the English name "Monday". For similar reasons the official liturgical calendar of the Roman Catholic Church refers to Monday as "Feria II". (The Portuguese name for Monday reflects this, as do all the days' names except Saturday and Sunday: the Portuguese word for Monday is segunda-feira.)