MyMetaBox-fix.php

<?php

class MyMetaBox {

  function __construct(){

    add_filter('rwmb_meta_boxes', array(&$this, 'my_meta_box'));
    add_action('wp_ajax_get_options', array(&$this, 'get_options'));

  }

  public function my_meta_box($meta_boxes){

    $select_advanced_ajax_id = "_my_select";
    
    $meta_boxes[] = array(
      'title'  => 'Infos',
      'post_types' => array( 'page' ),
      'fields' => array(
        array(
          // Script handle select2
          'std' => '
            <script type="text/javascript">
              jQuery(function($){
                var $interface = $("#'. $select_advanced_ajax_id .'_interface");
                var $field = jQuery("#'. $select_advanced_ajax_id .'");

                $field.on("change", function(){
                  var $this = $(this);
                  var value = $this.find(":selected").val();
                  var text = $this.find(":selected").text();
                  var data = "";
                  if(value != ""){ data = JSON.stringify({ value : value, text : text }); }
                  $interface.val(data);
                });

                if($interface.val() != ""){
                  var data = JSON.parse($interface.val());
                  var newOption = new Option(data.text, data.value, true, true);
                  $field.append(newOption);
                }
              });
            </script>',
          'type' => 'custom_html',
        ),
        array( // Hidden field
          'name' => 'My Super Real Select',
          'id'   => $select_advanced_ajax_id.'_interface',
          'type' => 'hidden',
          'sanitize_callback' => 'none',
        ),
        array(
          // Real field
          'name' => 'My Super Select',
          'id'   => $select_advanced_ajax_id,
          'type' => 'select_advanced',
          'sanitize_callback'   => 'none',
          'js_options' => array (
            'ajax' => array(
              'url' => '/wp-admin/admin-ajax.php',
                'dataType' => 'json',
                'type' => 'post',
                'data' => [
                  'action' => 'get_options',
                ],
                'delay' => '250',
              ),
            'minimumInputLength' => 0,
            'allowClear' => true, 
            'placeholder' => 'Select an option',
          ),
        ),
      ),
      
    );

    return $meta_boxes;

  }

  public function get_options(){

    $results = [
      'results' => [],
    ];

    for($i = 1; $i <= 10; $i++):

      $results['results'][] = array(
        // 'id' => $i,
        'id' => $i,
        'text' => 'Option '.$i,
      );

    endfor;

    wp_send_json($results);

    wp_die();

  }

}

new MyMetaBox();