converting from php3 to php4

I am in the process of converting from php3 to php4 and at the same time
going from mysql to postgres.

All the relevent files reside in

../foo/bar

And the vast majority are in the form *.php3 with internal references to
other webpages which (of course) are also in the form *.php3.

What I need is something that can go into that particular sub directory and
find every instance of php3 and make it php, both as a part of a file name
and internally in the file.

Thanks for the help.

Roy F. Cabaniss