Plugin code help

Hi all,

In a templete, when we want to display a link for the category and a comment link for each post, we write this code:

<?php the_category(‘, ‘) ?>
<?php comments_popup_link(‘No Comments »’, ‘1 Comment »’, ‘% Comments »’); ?>

Now instead of this, I want to write a plugin which can generate XML code instead of HTML code.

Can someone tell me what should this code look like?

From my understanding, I should be writing a plugin PHP file with some function name and calling that function in the template code.

If that is correct, does anyone know where I can get some help to find the right code to place in the plugin PHP file for Category and Comment links?

Thanks.