Yii2 PHP 7.1.3 - 调用未定义的函数yii \ helpers \ mb_strlen（）

I updated php from 7.0 to 7.1, and now I am getting this error:

Call to undefined function yii\helpers\mb_strlen()

I run this command (and restarted the server):

sudo apt-get install php7.1-mbstring

Got this message:

Reading package lists... Done
Building dependency tree       
Reading state information... Done
php7.1-mbstring is already the newest version (7.1.3+-3+deb.sury.org~xenial+1).
The following package was automatically installed and is no longer required:
  rlwrap
Use 'sudo apt autoremove' to remove it.
0 upgraded, 0 newly installed, 0 to remove and 36 not upgraded.

Tried everything, but the error is still here.

Screen shot: