Need advice on proper installation of PHP script

Hello! I am running a gaming community site for Guild Wars on Drupal 5.1. I am attempting to integrate gwbbcode, which is a script that aids in displaying skill builds. It was originally designed for phpbb, but it is adaptable. For a working example, please see this page:

http://gwshack.us/1ef3a

If you click on the "show gwBBcode" button at the bottom of that page, you will see the text that was converted into those lovely graphical "skill bars." The function that executes this function relies on specific tags, so it easy to integrate builds along with other plaintext. The function that performs the translation from simple text code to beautiful HTML code is called "parse_gwbbcode()", and it is found in a file called gwbbcode.inc.php.

In order to use the code, I hacked a template's node.tpl.php to include these lines:

<?php
define('GWBBCODE_ROOT','/home/lennalf/public_html/test/gwbbcode');
define('GWBBCODE_URL','http://test.lennalf.com/gwbbcode');
include GWBBCODE_ROOT.'/gwbbcode.inc.php';
?> 

and then replaced
<?php print $content ?>
with:
<?php $content2 = gwbbcodize($content); print $content2 ?>
(gwbbcodize points directly to parse_gwbbcode)

As a final step, I also edited page.tpl.php to include:

<?php echo
str_replace('{gwbbcode_root_path}','http://test.lennalf.com/gwbbcode',file_get_contents('/home/lennalf/public_html/test/gwbbcode/overall_header.tpl'));
?> 

... which pulls in the necessary javascript and styles.

I am sure some of you are wincing right now, because this is probably the ugliest hack in the history of Drupal.

Long story short, the result is actually mostly functional. You can see http://test.lennalf.com/ for an example of the code's output. However, if you were to sign up and create your own post (please do if you care to), you would find that there is a glaring error. When editing the post, if one attempts to "Preview" the revision, the following error occurs:

Fatal error: Cannot redeclare parse_gwbbcode() (previously declared in /home/lennalf/public_html/test/gwbbcode/gwbbcode.inc.php:30) in /home/lennalf/public_html/test/gwbbcode/gwbbcode.inc.php on line 30

I am assuming that this is because node.tpl.php is being called twice in order to present the preview, which means it is also trying to include gwbbcode.inc.php twice, which means the parse_gwbbcode() function is being defined twice, which is a no-no. This is no doubt a result of trying to shove the function into the node template.

So, essentially I've gotten to the point where I could continue to hack up the script until it barely makes sense, or I can find the way to do this right. I am hoping that somebody here in the Drupal community will see what I've done here and educate me on the errors of my ways. I am not very familiar with the Drupal API and I'm not too great with php, so I would really appreciate some advice from an expert on how to properly integrate this php script. Ideally, the script would only execute against "content," as I don't want anybody trying to throw gwbbcode in titles or names or something silly like that.

(Ideally, I would make a module, but I know so little about the Drupal API that such a task seems impossible right now)

I eagerly await your replies. Thanks in advance!