Description:
------------
The code highlight functions should output a surrounding <pre> element, 
as the content's meaning may be changed by normalising whitespace. A 
non-breaking space has a different meaning to whitespace having 
importance.

e.g., the two are different strings:
- 'This is a
test'
- 'This is a test'

Reproduce code:
---------------
<?php

highlight_string('foo  bar');

Expected result:
----------------
&<code><span style="color: #000000">
foo&nbsp;&nbsp;bar</span;
</code>

Actual result:
--------------
<pre><code><span style="color: #000000">
foo  bar</span>
</code></pre>