输入字符串，返回字符串前三字母

//函数，输入字符串，返回字符串前三字母
    public function getStoreName($str){
        $one = mb_substr($str, 0, 1, 'utf-8');
        $two = mb_substr($str, 1, 2, 'utf-8');
        $three = mb_substr($str, 2, 3, 'utf-8');
        
        if(preg_match('/^[\x7f-\xff]+$/', $one)){  
            $one = getFirstCharter($one);
        }
        if(preg_match('/^[\x7f-\xff]+$/', $two)){  
            $two = getFirstCharter($two);
        }
        if(preg_match('/^[\x7f-\xff]+$/', $three)){  
            $three = getFirstCharter($three);
        }

        return   $one.$two.$three;

    }

好像有点问题