Description:
------------
The code highlight functions should use <var> for variables in the 
highlighted code.

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

highlight_string('$foo');

Expected result:
----------------
<code><span style="color: #000000">
<var>$foo</var></span>
</code>

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