title: PHP Exec() or System() Causes 500 Error
body: <p>A fairly simple problem, I think. Here's my testing code (obviously I'll add security precautions when I'm writing this for real) -</p>
  
  <pre><code>&lt;?php
      if (isset($_POST['url']) &amp;&amp; !empty($_POST['url'])) {
          system('wget --no-parent --timestamping --convert-links --page-requisites --no-directories --no-host-directories -erobots=off '.$_POST['url']);
  } else {
      echo '&lt;form method="post"&gt;&lt;input type="url" name="url"&gt;&lt;input type="submit"&gt;&lt;/form&gt;';
  }
  
  ?&gt;
  </code></pre>
  
  <p>Is there some module in PHP or Apache2 that I need to enable/install for this to work? I'm visiting this page through the browser, if that helps. This happens even when the command has not been called yet (that part of the <code>if</code> statement is not run), and the 500 Error goes away if I comment out the system() or exec() line.</p>