Suggestions

Rather than give a less than stellar review, I figured I would point out some issues first so you have an opportunity to fix them. You should test with WP debug mode turned on so that you can see potential problems. I don’t think you are doing this or you would see the undefined index notices and fix that. Because you are not checking to see if $_POST['start'] and $_POST['end'] are defined, you get a notice when the admin screen loads. You can fix this by changing line 132 from this: $start = $_POST['start']; to this: $start = ( isset( $_POST['start'] ) ) ? $_POST['start'] : false; (Same thing for line 134 only with “end”.) The other element is a minor annoyance that other people have mentioned as well. CSV stands for “Comma Separated Values”. Your output delimits with a semi-colon. Sure, there are different delimiters that can be used in text to identify columns. But when you say “csv” people expect a csv format (which means comma). https://wordpress.org/plugins/export-users-data-to-csv/