Example Callback function for Ultimate Member dropdown field

example.php

function getCities() {
//get the value from the 'parent' field, sent via the AJAX post.
  $choice = $_POST['parent_option'];

  //Depending on the value of $choice, return a different array.
  switch($choice) {
    case "France":
      $cities = [
        "Paris" =>"Paris",
        "Marseille" => "Marseille",
        "Lyon" => "Lyon"
      ];
      break;
    case "Spain":
      $cities = [
        "Madrid"=>"Madrid",
        "Barcelona"=>"Barcelona"
      ];
      break;
    default:
      //code to do something if other options are not selected (throw an error, or set $cities to a default array)
      $cities = ["no city"];
      
  }
return $cities;

}