Problem with file uploads

I'm using:
- Apache 2.0.40 server (linux)
- PHP 4.2.2

I use file uploads in my code, but there occured small problems with the 
server update(from Apache 1.3.??, PHP 4.1.2). is_uploaded_file()- and 
move_uploaded_file()-functions don't work correctly any more. Or 
actually the file upload system doesn't work correctly.

The problem is to recognice, when the user try to upload a file, which 
doesn't exist. It's possible to even write nonsense like "sdafasdf" into 
the file type input-field (Mozzilla and IE). In that case the current 
server writes an empty file named according to the text "sdafasdf".

When there is a legal empty file "uploaded" to the server, 
is_uploaded_file() doesn't fail. So, there is no way to recognice, when 
the user try upload a "nonsense-file".

Can anybody help? I don't know, if it's PHP or Apache causing the 
problem. Is there any settings, which could cause this?