wowser - I just typed 'image manipulation class' into good old google and guess who is at number one? - well, via resourceindex
as to the question:
you would also need the truetype libraries compiled in the GD build (unless you just want the 5 default fonts - tnr, arial etc) - best ones are from freetype.org
useful functions to check in the manual
imagestring() - using default fonts
imagettfbbox() - get bounding area
imagettftext() - write string
try a simple .ttf image write first (bound to be one at php.net) to check whether you can access user font files properly - after that the function coding should be straight forward enough.
mutters 'wowser' again.