How do I sort posts with multiple pages

You’re initially submitting a POST request via a form with the sort parameters. When you click links to additional pages, you’re just sending a GET request for the next page without those original POST vars, so they don’t carry over to the additional pages and aren’t picked up by your if(isset($_REQUEST['sort'])). Probably the simpler way to handle it is to pass the sort parameter as a query string appended to the URL, then check for that query string and append it to the URLs of the additional pages.