title: php bin/console symfony commands give a fatal error
body: <p>I installed a symfony 3 project but cant run any php bin/console commands. When I try, I get the following error:</p>
  
  <blockquote>
    <p>PHP Fatal error:  Uncaught Symfony\Component\Debug\Exception\ClassNotFoundException: Attempted to load class "DOMDocument" from the global namespace.
    Did you forget a "use" statement? in project_folder/vendor/symfony/symfony/src/Symfony/Component/Config/Util/XmlUtils.php:52</p>
  </blockquote>
  
  <p>Any idea how to fix this?</p>