I'm having a seemingly very simple problem.
I'm building a store with Yahoo! merchant from the ground up--not using any of their templates, etc. I using a small amount of PHP (only) to use includes. However, the problem doesn't appear to be with the PHP code, rather the .php extension.
Problem:
When I name a file "index.htm", the store tags provided by Yahoo! work perfectly. But, when I name a file "index.php", the store tags break. It looks as though it as to do with the structure of the Yahoo! store tag code.
Here's an example:
http://devilinthewoodsestore.com/fiver/test.htm (works)
http://devilinthewoodsestore.com/fiver/test.php (broke)
The code for both of these files is identical--the only difference is the file extension. Like I mentioned, I believe it has something to do with the structure of the Yahoo! store tag.
Here's the source of the two mentioned pages:

Any ideas on how I can fix this bug and still get the Yahoo! store tags to function properly?
I'm new to PHP, so any guidance would be especially appreciated!
Thanks!
-Sabrina