Just say "free alternative", you don't care about open source you just want the free part. However, you could find a lot of systems out there that say they can encrypt and protect your code. They are nothing more then a joke, don't bother using them. IonCube and Zend encrypt the bytecode PHP interpreter returns that is what you want.
Since you are here looking for a "free alternative" that must mean the code you are trying to protect is not that valuable. If it was you would be more inclined to invest in its protection.