MyMetaBox-fix2.php

<?php

class MyMetaBox {

  function __construct(){

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

  }

  public function my_meta_box($meta_boxes){

    $select_advanced_ajax_id = "_my_select";

    $options = [];
    if(is_admin() && isset($_GET['post']) && get_post_type($_GET['post']) == 'page'):
      $my_options = $this->get_my_options();
      foreach($my_options as $option):
        $options[$option['id']] = $option['text'];
      endforeach;
    endif;
    
    $meta_boxes[] = array(
      'title'  => 'Infos',
      'post_types' => array( 'page' ),
      'fields' => array(
        array(
          // Real field
          'name' => 'My Super Select',
          'id'   => $select_advanced_ajax_id,
          'type' => 'select_advanced',
          'options' => $options,
          'sanitize_callback'   => 'none',
          'js_options' => array (
            'minimumInputLength' => 0,
            'allowClear' => true, 
            'placeholder' => 'Select an option',
          ),
        ),
      ),
      
    );

    return $meta_boxes;

  }

  public function get_my_options(){

    $results = [];

    for($i = 1; $i <= 10; $i++):
      $results[] = array(
        'id' => $i,
        'text' => 'Option '.$i,
      );
    endfor;

    return $results;

  }

}

new MyMetaBox();