How to add an HTML comment to the head of every page?
You're better off using a more Drupal-centric solution. If you're looking to place some static copyright info, you can create a block of HTML and place it in a footer region of your site's theme (under the block administration area). 

If you're just looking to play around with the Drupal CMS &amp; PHP coding in general (off topic for Drupal Answers), as long as you make your changes outside of the &lt;?php ?&gt; tags you should be fine to make changes to your template file.