tp框架写了个步骤获取顶级分类return结果就是null，在辅助函数都能打印出结果

     //通过id_category获取一级分类id
     function getParentInfo(){
         $id_category=Tools::getValue('id_category');
         $cate_info=$this->category_obj->where('id_category='.$id_category)->find();
         //不是顶级
         if($cate_info['pid']!=0){
             $result=$this->getFirst($id_category);
         }else{
             $result=$cate_info;
         }
         dump($result);
     //    dump('顶级类是:'.$result['id_category'].':'.$result['name'].'
');
     }    
    function getFirst($id_category){     
          $cate_info=$this->category_obj->where('id_category='.$id_category)->find();   
          echo $this->category_obj->getLastSql().'
';  
          if($cate_info['pid']!=0){
               $this->getFirst($cate_info['pid']);               
          }else{
               dump($cate_info);
               return $cate_info;
          }          
     }

function getFirst($id_category){   
  $cate_info=$this->category_obj->where('id_category='.$id_category)->find();   
       echo $this->category_obj->getLastSql().'
';   
  if($cate_info['pid']!=0){
       $cate_info =$this->getFirst($cate_info['pid']);   
  }
      return $cate_info;
    }

------解决方案--------------------
要有返回值!