Text file --> imagepng()

I have a text file, how can I make the text a png image?

thanks

text as in text or text as in binary string?

$a = imagecreatefromstring('file.ext');
header("Content-type: image/png");
imagepng($a);

other than that, check the functions

imagestring
imagestringup
imagettfbbox
imagettftext
imagefttext
imageftbbox

in the manual - presumes compiled GD module.