Almost all of us would have used PHP to design websites with dynamic text, but I imagine far fewer would know that generating images on the fly isn't all that difficult either. In fact, there's a library available for doing this very task -
GD - and it's no surprise to find that there are
PHP bindings for it.
Developworks have recently published an extensive tutorial on
using the GD library with PHP (requires free registration). It covers not only basics such as drawing pixels, lines and circles, but also touches on applying effects to existing images, and drawing text (good for making
captchas).