protected function subtree($arr,$id=0,$lev=1){
    $subs = array(); // 子孙数组
    foreach($arr as $v) {
      if($v['parent_id'] == $id) {
        $v['lev'] = $lev;
        $subs[] = $v; // 举例说找到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 — 重复一个字符串
    }
    $arr=array(
      'new_cat_name'=>'顶级分类',
      'cat_id'=>0
    );
    array_unshift($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' => '请选择父级分类']) ?>