You can populate the comment box w/ data by putting the data into a $variableWithData, then using that variable as the value="" attribute of the comment box (which is typically just an html form).

The value attribute would look like this:

value="<?php echo $variableWithData; ?>"

and just fits into the textarea tag as per usual. The pages have to be able to parse php. Users will be able to replace the data in the comment box with their own text. This technique is also useful for filling in form fields that users have already completed when returning them to a form for corrections.