There is a code with errors, you need to fix it. The code is only 1 page. I can’t show it here, because. I bought it and therefore I will violate copyright.
PHP Notice: Undefined variable: nxt in
PHP Notice: Uninitialized string offset: 0 in
You may also find other errors.
Answer 1, authority 100%
I think it’s enough to insert at the beginning of the script
$nxt = '';
This will explicitly state that the variable exists before using it.
Answer 2, authority 33%
Notice is not an error, an error is not a warning. In general, I advise you to contact the author, because it’s not good to sell code of this quality.