debug to file

gistfile1.txt

if(\Drupal::service('kernel')) { // make a conditional test for debug or not
  ob_start();
  // insert var below
  var_dump(true);
  // insert var above
  $data = ob_get_clean();
  $fp = fopen("textfile.html", "a");

  // write $data or custom string, ie "I'm here".
  fwrite($fp, "true");
  fclose($fp);
} else {
  $fp = fopen("textfile.html", "a");

  // write $data or custom string, ie "I'm here".
  fwrite($fp, "false");
  fclose($fp);
}