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

Question

我将php从7.0更新到7.1，现在我收到了这个错误：

Call to undefined function yii\helpers\mb_strlen()

我运行此命令（并重新启动服务器）：

sudo apt-get install php7.1-mbstring

收到此消息：

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.

尝试了一切，但错误仍然存​​在。

截屏：