change thread sort order?

[bluestartech]

Excuse my lack of knowledge in this respects. I would like to make it into a plugin like this, but I do not know how to implement the php code section..

<?xml version="1.0" encoding="ISO-8859-1"?>
<plugins>
<plugin active="1">
<title>Reverse Thread Sorting</title>
<hookname>showthread_getinfo</hookname>
<phpcode><![CDATA[$this->validfields['forumhomeicon'] = array(TYPE_STR, REQ_NO);]]></phpcode>
</plugin>
</plugins>

if($threadid == '35') $postorder = 'ASC';

And then do I have call the hook in another file or will it work as an option automatically?

Thank you