When I run composer install inside CakePHP3 I get extension missing errors
<p>When I run <code>composer install</code> inside my CakePHP3 project I get next errors:</p>

<pre><code>  Problem 1
    - Installation request for cakephp/cakephp 3.2.12 -&gt; satisfiable by cakephp/cakephp[3.2.12].
    - cakephp/cakephp 3.2.12 requires ext-mbstring * -&gt; the requested PHP extension mbstring is missing from your system.
  Problem 2
    - cakephp/cakephp 3.2.12 requires ext-mbstring * -&gt; the requested PHP extension mbstring is missing from your system.
    - cakephp/debug_kit 3.2.9 requires cakephp/cakephp &gt;=3.1.0 &lt;4.0 -&gt; satisfiable by cakephp/cakephp[3.2.12].
    - Installation request for cakephp/debug_kit 3.2.9 -&gt; satisfiable by cakephp/debug_kit[3.2.9].
</code></pre>

<p>I am using Ubuntu 16.04 (note disabled packages for PHP5)</p>

<p>How do I fix this issue?</p>