title: Call to undefined function mb_strlen() on PHP 7 Ubuntu 14.04
body: <p>I had a Yii installation that was working fine up until today. I installed updates on my machine today after which this error was being thrown on all pages:</p>
  
  <blockquote>
    <p>PHP Fatal Error – yii\base\ErrorException</p>
    
    <p>Uncaught Error: Call to undefined function yii\web\mb_strlen() in /application/vendor/yiisoft/yii2/web/ErrorHandler.php:404</p>
  </blockquote>
  
  <p>I ran <code>composer update</code> but the error persists.</p>
  
  <p>I am using yii 2.0.7 on PHP 7 on Ubuntu 14.04. How do I fix this?</p>