Can't find .php3 files

I'm sure this is a FAQ but I can't seem to come up with the right search keys to dig it out. I'm trying to help a friend migrate his application to php 5 from another system. The problem seems to be that he references files (require, include, etc) that have a .php3 extension, however there are no files in those locations with the .php3. There are files with .php extensions. It's running on a system that has php 4 on it. So I'm sure either php or Apache is rewriting the files somehow, but I don't know how. Can someone please point me to the appropriate documentation or give me a hint as to what is going on here? Thanks, Jim.