csv-generator | small utility to generate millions | CSV Processing library

The error message tells you that your variable (tags, filter_field, filter_country) does not exist. This means that the variables in question do not exist at the point when you try to refer to them. Things to work on to ensure this is fixed.

You should have the file containing the definition of those variables required/included. It's easy to test whether that's the case: just throw an error (not on prod, if possible) from the file that you assume to be already existent and another from the file you are trying to use it at the point where you try to use it. If the first error is thrown, then the file is evaluated at the point you try to use it. If not, then not.

Try to throw an exception from the inside of the if. If it's thrown, then export_rule is set in $_POST.

Some of your items might be missing from $_POST. It is recommended to use defaults for the case when they do not exist. Your HTML looks to have some form tags without the items that you expect. You need to have some HTML tags with their name having the value of tags, filter_field and filter_country respectively inside the form tag. If you do not have that - and that looks to be the case - then that's a problem to fix for sure.

Make sure you close your tags properly. Use an HTML validator (like this one: https://validator.w3.org/) to check what makes your HTML invalid and fix the issues.