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

I encountered a problem today when writing a custom mvc framework:

SQLSTATE[HY000]: General error: mode must be an integer

Paste the code below:

//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);
        }
    }

Code prompt: General error: mode must be an integer, I checked the document again

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

The mark here is that $fetch_style must be of type int , then I directly changed $this->fetch_mode directly to PDO::FETCH_ASSOC, and found that the operation was successful, and then flipped to the one stored in my config It turned out to be a string type:

'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'=>''

    ),

But for the sake of beauty, I decided not to change config, so......................:

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

Document explanation:

constant    ( string $name   ) : mixed

Returns the value of the constant through name .

constant() is very useful when you don’t know the name of the constant but need to get the value of the constant. That is, the constant name is stored in a variable, or the constant name is returned by a function.

Return the value of the constant. Returns NULL if the constant is undefined.

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