you're using two vars as it says.
base64_encode($_FILES['userfile']['
tmp_name'], $uploadfile)
In this case you can only check one of them but in you can do an if AND thing.
if(chunk_split(base64_encode($_FILES['userfile']['
tmp_name'])) AND chunk_split(base64_encode($uploadfile)) )
[url]http://us3.php.net/manual/en/function.base64-encode.php[/url]
That is my shot at it ;-p