title: Yii2 PHP 7.1.3 - Call to undefined function yii\helpers\mb_strlen()
body: <p>I updated php from 7.0 to 7.1, and now I am getting this error:</p>
  
  <pre><code>Call to undefined function yii\helpers\mb_strlen()
  </code></pre>
  
  <p>I run this command (and restarted the server):</p>
  
  <pre><code>sudo apt-get install php7.1-mbstring
  </code></pre>
  
  <p>Got this message:</p>
  
  <pre><code>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.
  </code></pre>
  
  <p>Tried everything, but the error is still here.</p>
  
  <p>Screen shot:
  <a href="https://i.stack.imgur.com/5cDnw.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/5cDnw.png" alt="enter image description here"></a></p>