Mode must be an integer appears when running PHP. How to solve it?

//这是报错位置的代码
public function dao_query($sql,$all = true){
        try{
            $stmt = $this->pdo->query($sql);

            //设置fetch_mode
            
            
             $stmt->setFetchMode($this->fetch_mode);//这里报错
            //解析数据
            if(!$all){
                return $stmt->fetch();
             }else{
                return $stmt->fetchAll();
               }
            }catch(PDOException $e){
            $this->dao_exception($e);
        }
    }

PDOStatement::fetchAll([ int $fetch_style [, mixed $fetch_argument   [, array $ctor_args = array()  ]]] ) : array

'database'=>array(
         'type'=>'mysql',
         'host'=>'localhost',
         'port'=>'3306',
         'user'=>'root',
         'pass'=>'root',
         'charset'=>'utf8',
         'dbname'=>'my_database',
         'fetch_mode'=>'PDO::FETCH_ASSOC',//这里的问题
         'prefix'=>''

    ),

       $stmt->setFetchMode(constant($this->fetch_mode));//这里必须使用int $model  constant（$this->fetch_mode）

constant    ( string $name   ) : mixed