AJAX と PHP を使用してファイルを効果的にダウンロードするにはどうすればよいですか?

function csv() {
  ajaxRequest = ajax();
  postdata = "data=" + document.getElementById("id").value;
  ajaxRequest.onreadystatechange = function () {
    var ajaxDisplay = document.getElementById('ajaxDiv');
    if (ajaxRequest.readyState == 4 && ajaxRequest.status == 200) {
      document.location = 'download.php?filename=' + ajaxRequest.responseText;
    }
  };
  ajaxRequest.open("POST", "csv.php", false);
  ajaxRequest.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
  ajaxRequest.send(postdata);
}

$fileName = 'file.csv';
$downloadFileName = 'newfile.csv';

if (file_exists($fileName)) {
  header('Content-Description: File Transfer');
  header('Content-Type: text/csv');
  header('Content-Disposition: attachment; filename='.$downloadFileName);
  ob_clean();
  flush();
  readfile($fileName);
  exit;
}