<p>Assuming the following directory structure,</p>

<pre><code>htdocs/
  images/
  css/
  .htaccess
system/
  index.php
  ...
</code></pre>

<p>I would like to route all incoming requests through that php script. I have been trying some rewrite rules within the htaccess, but I can't seem to be able to route to files that are outside the document root. I couldn't find a reason for this in the apache manuals, so eventually resorted to "Include" an apache config file, where the rules are different.</p>

<p>Is there a way of rewriting outside of the docroot?</p>

No, you cannot route outside the docroot without potentially enormous risk.
You should place the
```
index.php
```
within the
```
htdocs
```
folder and rewrite it there, not outside.  If you were to rewrite to handle the index, accessing outside the docroot, and you made one little mistake it's potentially exploited by any hacker rewriting the request string.  Not pretty!