Zend Guard seems to be the de facto standard. However, a quick google search revealed a couple others:
- SourceGuardian
- PHP LockIt
- PHP Cipher
In the long run, though, all of these just end up pissing off the people who have to make modifications to their server to run a script. My advice, as someone who has both used and released scripts with this type of encryption, is that you not bother, because someone will (given enough time) decrypt it anyways.
