title: Symfony 3 composer, ext-dom and ext-xml missing
body: <p>I am running Linux Mint 18.1. My IDE is PhpStorm and I am trying to install PHPUnit trough the composer. I followed the installation steps at <a href="https://getcomposer.org/download/" rel="noreferrer">https://getcomposer.org/download/</a>.</p>
  
  <blockquote>
    <p>Problem 1
        - Installation request for phpunit/phpunit 6.4.x-dev -> satisfiable by phpunit/phpunit[6.4.x-dev].
        - phpunit/phpunit 6.4.x-dev requires ext-dom * -> the requested PHP extension dom is missing from your system.<br>
    Problem 2
        - symfony/symfony v3.3.6 requires ext-xml * -> the requested PHP extension xml is missing from your system.
        - symfony/symfony v3.3.6 requires ext-xml * -> the requested PHP extension xml is missing from your system.
        - symfony/symfony v3.3.6 requires ext-xml * -> the requested PHP extension xml is missing from your system.
        - Installation request for symfony/symfony (locked at v3.3.6, required as 3.3.*) -> satisfiable by symfony/symfony[v3.3.6].</p>
  </blockquote>
  
  <p>Solutions I have tried:</p>
  
  <ul>
  <li>Install php-xml</li>
  <li>Install php7.0-xml</li>
  <li><a href="https://stackoverflow.com/questions/37873482/laravel-composer-update-the-requested-php-extension-dom-is-missing-from-your-s">This answer</a> says to check if "extension=dom.so" is enabled in any of your php.ini files. It is not even present in my files</li>
  </ul>
  
  <p>Any suggestions? Thanks a lot!</p>