Survey said! Wolf Fence in Alaska.
The basic idea is that you divide your problem space in half by inserting a
print "Hi, Mom!\n"; exit; (insert your favorite phrase) somewhere near the "middle" of your code. If you get the message, then the bug is beyond where you put the print, so move it farther along in the execution. If you don't get there, move the print earlier.
Lather, rinse, repeat.
If you Choose Wisely about where to put the print you can narrow a 1,000,000 line program down to 1 line in just 20 tries.
This is faster/easier to do from the command line, but it's possible to do it via FTP.