While one would think that the 'rename' command would have a recursive option, it (for some reason, I hope is good) does not.
So, rather than typing...
rename -r *.php *.php4
One must type...
find . | sed 's/\(.*\)\.php/mv \1.php \1.php4/'|sh
which works beautiful, thought it's one of the uglier command line statements I've seen.
Special thanks to Brad Garrett for the majority of the line. I added the '.' after 'find'. :)