Read the first four links at
http://us3.php.net/features.file-upload
Especially important is the second link,
http://us3.php.net/manual/en/feature...oad.errors.php
To debug your code, you need to use the information from the $_FILES array
Until your code includes comparisons with at least the UPLOAD_ERR_OK constant, you're probably not dealing with the PHP file upload status.
When uploads fail in PHP, they won't necessarily generate warnings or errors, but do read about those file upload errors (examples are provided). Doing this should help provide more information about your problem.