title: PHP's HTTP_IF_NONE_MATCH is always empty
body: <p>PHP's <code>$_SERVER['HTTP_IF_NONE_MATCH']</code> is <em>always</em> empty however Firefox's Web Developer Network Requests tab shows the <em>request header</em> as being <code>If-Modified-Since: "Tue, 27 Jun 2017 09:08:23 GMT"</code>. Cache is <em>not</em> disabled and my <code>.htaccess</code> file contains the following:</p>
  
  <pre><code>RewriteEngine on
  RewriteRule .* - [E=HTTP_IF_NONE_MATCH:%{HTTP:If-None-Match}]
  </code></pre>
  
  <p>How do I get PHP's <code>$_SERVER['HTTP_IF_NONE_MATCH']</code> to return the request header properly <em>or</em> how do I access that request header in an alternative manner? I came across <code>getenv('HTTP_IF_MODIFIED_SINCE')</code> though that didn't return anything either. Is it possible there is something in the <code>php.ini</code> file that needs to be changed? It is a fairly fresh server setup.</p>