protected function subtree($arr,$id=0,$lev=1){    $subs = array(); // 子孫數(shù)組    foreach($arr as $v) {      if($v['parent_id'] == $id) {        $v['lev'] = $lev;        $subs[] = $v; // 舉例說找到array('id'=>1,'name'=>'安徽','parent'=>0),        $subs = array_merge($subs,$this->subtree($arr,$v['cat_id'],$lev+1));      }    }    return $subs;  }public function actionCreate()  {    $model = new EcsCategory();    $query = new /yii/db/Query();    $query->select('*')      ->from('ecs_category');    $command = $query->createCommand();    $res=$command->queryAll();    $tree = $this->subtree($res,0,1);    foreach($tree as $k=> $v) {      $tree[$k]['new_cat_name']=str_repeat('--',$v['lev']).$v['cat_name'].str_repeat('--',$v['lev']); //str_repeat — 重復一個字符串    }    $arr=array(      'new_cat_name'=>'頂級分類',      'cat_id'=>0    );    array_unshift($tree,$arr);    if ($model->load(Yii::$app->request->post()) && $model->save()) {      return $this->redirect(['view', 'id' => $model->cat_id]);    } else {      return $this->render('create', [        'model' => $model,        'data'=>$tree,      ]);    }  }

 use /yii/helpers/ArrayHelper;<?= $form->field($model, 'parent_id')->dropDownList(ArrayHelper::map($data,'cat_id','new_cat_name') ,['prompt' => '請選擇父級分類']) ?>