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?

Well, it makes much more sense. Either Monday or Saturday should be the first day of the week. I.e. 1 week = workweek + weekend or weekend + workweek. Now, weekEND would suggest the first, i.e. Monday.

Choosing Sunday is as retarded as choosing Wednesday. I don't care what day God allegedly rested. Changing from Sunday to Monday was a much appreciated bugfix. US should update.