or you could try this:
http://www.phpmix.org/how_to_create_a_dynamic_signature
It may be a bit different to what you want, but it'll do a similar job. Just put whatever code you want in the 'text' thingy and away you go.
$image_text = array(
array(
'x' => 8,
'y' => 6,
'color' => array(50, 100, 180),
'font' => 9,
'text' => $anythinghere
),
I have created this using SMF stats and whatnot in variables:
in the url you can set variables like userid, colour of the background and style etc with some simple code (e.g. http://www.com/sig.png?user=71&colour=green etc
The Username, number of posts, and user is [not] online thing is all done with php.