You could have a look at Turck mmCache, it stores the compiled PHP scripts in a cache folder. It can run scripts from that folder too, so you can distribute your compiled PHP scripts instead of giving away the source.
I was mostly looking for ways to run it more efficiently (not have to parse the code every time the script is called), though encryption would also be useful.
Do you know of a site where I could find more information on Turck mmCache?
Turck mmCache is a massive boost on php performance for me.
On phpBB this couples well with the extreme themes mod (all themes are instead of being loaded and dynamicaly executed) turned in php files and directly compiled, making it extremly zippy