Well, for starters, show us your code, and let us know a tad about the system it's on, if it's not obvious from reading your code.
Secondly, in the future, try not to hijack other people's threads 🙂 ...
Now, to the problem at hand ... if the "temp" file is not an uploaded file, could it be possible that it's not being placed in the temporary location? That's the primary reason why an if call to [man]is_uploaded_file[/man] would fail ... unless, of course, your browser is simply not POSTing the data correctly...
What is your temp dir (check php.ini...)? If you're on some 3rd party hosting site, is it possible that you don't have access to the PHP default temp dir?
Also, be sure and check what Adam told you. It's possible that by using a bad pathname (for example, no trailing slash) you could put a file where you don't want to...