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You need some code encoder. 

Stealed from somewere in net (pretty old info, but it gives the direction for search): 

- Source Defender
- PHP Screw
- Zend Encoder
- PBC
- PHTML Encoder
- NIA Encoder
- microCode
- POBS

But it's not a all time solutions. All files can be decompiled with more or less time and knowledge.