In a post on Sitepoint Christopher Pitt demonstrates some cool usages of eval and exec.

I want to look at two PHP functions: eval and exec. They’re so often thrown under the sensible-developers-never-use-these bus that I sometimes wonder how many awesome applications we miss out on. Like every other function in the standard library, these have their uses. They can be abused. Their danger lies in the amount of flexibility and power they offer even the most novice of developers. Let me show you some of the ways I’ve seen these used, and then we can talk about safety precautions and moderation.