I don't know of anything offhand....staring at PEAR๐ate might reveal something - in fact, staring at PHP's implementation of date() in the source code may locate just PHP does it.
Hm. Could be worth a play around..... I wonder if you could start with date('W') and do some arithmetic with that...?
/Weedpacket wanders off muttering to himself. We probably won't hear from him again for a while.