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?

In each row of a table, I could keep the date of that week's Monday. The Monday is gotten when the row is inserted. Upon entering the site, the rows with the same date are queried, simplifying (and possibly expediating) the execution of the site.

The trick is in determining the Monday. I can find the week-number for a date: but how do I find the first date of a week-number?