Here are a couple links to check out.
http://www.scriptol.com/
Thread on zend.com that speaks of the existence of a PHP compiler
maybe worth looking into?
Zend also has a solution to the problem you are facing. They have a tool that will "encode" PHP, which they claim "protects source code from copyright infringement". The downfall is that to run the "encoded" php you have to install the zend optimizer (free). If you were ambitious you could create an installer that would install the zend optimizer and your encoded php all at once, making it possible to easily install and run a PHP app on a client machine. It's somewhat similar to Java... you have code the means nothing unless run under a "byte code interpreter" (java virtual machine)... so to make it run, you install both your code, and the machine to run the code. Anyway, if you want to check that out go here: http://zend.com/store/products/zend-encoder.php .
I haven't personally used any of these, but I'd be very interested in hearing what you find out about any or all of them... especially scriptol, if it works well I think it could be very cool.