Adding PHP to single comments

Hello,

First, thanks for the new theme. I'm excited to give it a whirl.

The one problem I have is that I have a bit of PHP work I need to do for each comment, like so:

Use PHP to set variables

Print top of comment
Print variable set by PHP
Print body of comment
Print variable set by PHP
Print bottom of comment

I know I can't use PHP directly in the Montezuma options box, but I can't for the life of me figure out where to put it to achieve this.