When u generate your xml and insert code just do it like this:
$str = "<?php\r\n";
$str.= "\t/* my code */\r\n";
$str.= "?>\r\n";
$xml = highlight_string($str);
and this will be the output
<code><font color="#000000">
<font color="#0000BB"><?php
<br /> </font><font color="#FF8000">/* my code */
<br /></font><font color="#0000BB">?>
<br /></font>
</font>
</code>