How to install GMP for PHP7 on Ubuntu
<p>I was getting the following error when trying to run <code>composer update</code> on a SimpleSAML project.</p>

<pre><code>- openid/php-openid dev-master requires ext-gmp * -&gt; the requested PHP extension gmp is missing from your system.
</code></pre>

<p>Running <code>sudo apt-get install php5-gmp</code> did not work and neither did <code>sudo apt-get install php7-gmp</code></p>