/** @return string[] */
 private function formatAllMilestonesAsSelectboxOptions(array $planning_trackers_ids, $selected_milestone_id, Tracker $backlog_tracker, PFUser $user)
 {
     $hp = Codendi_HTMLPurifier::instance();
     $options = array();
     $current_milestone = array();
     $options[] = $this->addTopBacklogPlanningEntry($selected_milestone_id, $backlog_tracker, $user);
     foreach ($planning_trackers_ids as $id) {
         $current_milestone[$id] = null;
     }
     foreach ($this->dao->getAllMilestoneByTrackers($planning_trackers_ids) as $row) {
         foreach ($planning_trackers_ids as $index => $id) {
             $milestone_id = $row['m' . $id . '_id'];
             $milestone_title = $row['m' . $id . '_title'];
             if (!$milestone_id) {
                 continue;
             }
             if ($current_milestone[$id] === $milestone_id) {
                 continue;
             }
             $content = str_pad('', $index, '-') . ' ' . $hp->purify($milestone_title);
             $options[] = $this->getOptionForSelectBox($selected_milestone_id, $milestone_id, $content);
             $current_milestone[$id] = $milestone_id;
         }
     }
     return $options;
 }