I've used SourceGuardian, too, but I think I dropped it due to the fact that it caused more problems than it solved. In the end I thought, pssh, if people want to steal my source code, let them go for their life. Though Bluesaga (Bless him) will tell you a nightmare-like story of a system of mine and Karl's he bought from Sammy - safely encrypted with SourceGuardian. We still giggle about it now! And even those me and Bluesaga are good friends now, I'm still holding him to ransom over the source code :) !
The problem with these encryption algorithms is that they are required to be reversed to be interpreted by PHP. The individual who has the files that the encryption requires to be able to decrypt the code, and the encrypted files themselves, have all they need to decrypt the encrypted source code right there on their server. It's not like MD5 or SHA1 where the encrypted items do not need to be reversed. And as you see they need to be decrypted every time they are executed, so yes, they're easy to crack, but the speed to decrypt them is also a big drawback and a factor that should be taken into consideration.
The man who comes back through the Door in the Wall will never be quite the same as the man who went out.
I've never needed to encrypt my source code, since most of the time I work for customers whom pay for my development and source code. The other few occasions I didn't bother to encode it, since these customers always return to me for updates and new development.
But it's interesting to know what's out there and what the benefits are in that rare occasion where I might need it.
Don't worry, I won't encrypt my postings here since I believe it's also good to share knowledge and improve development coding by using usable code snippets and real situations.