Simple code obfuscation

Hey all - I'm selling a custom PHP solution to FileMaker users. It uses FileMaker's PHP API, so everyone who has FileMaker Server is already set up to use it, but very few of them have any knowledge of how to set up a server or do anything PHP related. But I do want to add some level of code obfuscation to prevent them from making simple changes to my code that allow them to exceed the privileges they've purchased. I've looked at custom code encryption services like Ioncube and phpCipher, but in my estimation, deploying the needed server-side code for these is going to be beyond the capabilities of a large segment of my customers. I would rather have a few customers cheat me than offer a product that most customers are unable to figure out how to run. So I was thinking of doing something like base64_encoding the crucial chunk of my code (maybe 20 lines worth) and using eval(base64_decode($that_content)) to run it. I figure that will scare away most of the customers who might be able to edit my code. Can anyone suggest something that goes one better? - Brian