I looking a software to encrypt source code. Below links is popular php encoder
I recommend Zend Guard or ionCube PHP Encoder because they are market player

PHP encoder
Zend Guard - $995 - Annual
ionCube PHP Encoder $199 - $348

SourceGuardian $250 - $450
CodeLock $35 USD one-time payment

Which one is for me?
Visit encodercompare.com

How it work?
Most encoder software use
- Obfuscation
- Compiler to native code or byte code
- Encryption