so you make a form with a text box on it name the text box Username then set the form to submit to another page via POST.
create a standard image for the sign and store it somewhere on the server.
then you would do something like this in the form handler page
// set these
$tc = array(255, 0,0);
$imgsrc = "images/sig.jpg";
$textx = 10;
$texty = 10;
$textsize = 12;
$font = "/usr/X11R6/lib/X11/fonts/TTF/luxisr.ttf";
// create a GD image from the base image
$img = imagecreatefromjpeg($imgsrc);
// add your text colour to the image palette
$colour = imagecolorallocate($img, $tc[0], $tc[1], $tc[2]);
// write the text
imagettftext($img, $textsize, 0, $textx, $texty, $colour, $_POST["Username"]);
// output image here
imagejpeg($img, "finishedsig.jpg");
echo("ALL DONE see image - finishedsig.jpg");
Does this make sense?