Most servers don't have a function to upload an entire PHP file, all the server app will let you do is communicate with it via forms. Unless you know the administrator's password.
Also PHP files might be communicating with a MySQL database, and gaining access to those would be just as challenging as gaining access to, say, BHW's database that has all the user names and passwords stored.
I'll take a look at the site if you want though. If the PHP file is supposed to preform a function that is rather simple, perhaps I could substitute it with a compatible one.
EDIT: PM me as I'm rather busy at the moment.