sample code snippets writing text to images?

I'm learning how to work with images in PHP and specifically learning how to write text dynamically to an existing image. Does anyone have any sample code and/or pointers that might help me out? I work best from examples, rather than muddling through the manual (though that's what I'm doing right now.) Ideally, if someone has an example of a reusable function to write text to existing images, that would be great. Thanks. --kurt