Reported post email -- add poster's username

[makaiguy]

Am trying to modify the reportpost phrase to include in the notification email the name of the person who submitted the post that is being reported.

I've tried:

Posted by:
$postinfo[username]

.. and a number of other variations, but have not found anything that works.

$reportinfo[postid] appears to contain the postid number for the reported post. How do I use this to return the username of the poster?