Problems with uploading file

I have the page where the user can enter information into text fields as well as upload a file all on the same form.  I have it setup in the following way:

First, I validate the file with a function, which checks to see if it is the right size, and the right type.

      function validate_file($files_array){
            $errors = array();
            $userfile_error = $files_array['userfile']['error'];
            $userfile_type = $files_array['userfile']['type'];
            
            if($userfile_error > 0){
                  switch($userfile_error){
                        case 1: $errors[] = 'File exceeded upload_max_filesize'; break;
                        case 2: $errors[] = 'File exceeded max_file_size'; break;
                        case 3: $errors[] = 'File only partially uploaded'; break;
                        case 4: $errors[] = 'No file uploaded'; break;
                  }
            }
            
            if($userfile_type != 'application/x-zip-compressed'){
                  $errors[] = 'File is invalid';
            }

            return $errors;
      }

Next, I validate the text fields.  If there are any errors in either of these functions, I display them in the same script.  Otherwise, I put all values into the session and do a header(location: ) to the preview script which just shows all the info.  Finally, I add all the text fields to the database, and finish the upload with the following functions, whihc gives me the problem:

      function upload_file($new_filename, $userfile){
            $errors = array();
            $filedir = '/home/sites/www.mysite.com/questions/';
            print $userfile . '<br>';
            $userfile_name = $new_filename;
                        $upfile = $filedir . $userfile_name;
            if(is_uploaded_file($userfile)){
                  if(!move_uploaded_file($userfile, $upfile)){
                        print 'Could not move file to destination directory';
                        //return $errors;
                  }
            }
            else{
                  print 'Possible file upload attack. Filename: '.$userfile_name;
            }
      }

where userfile is just the value of $_FILES['userfile']['tmp_name'] in a $_SESSION variable.  and new filename is the final name of the file.  This always fails and gives me the error 'Possible file upload attack' as shown above.  Does anybody know what the problem is?  The file doesnt even seem to be uploading to the tmp directory.  Are there any examples out there of a script which uploads a file as well as adds text input fields to a DB?  Thanks.