How to make an encoder for PHP?

I would like to know how to code an encoder like Zend Guard or SourceGuardian especially for PHP source code. What topics should I look after about this matter ? thx for helping
How to make an encoder for PHP?
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I would like to know how to code an encoder like Zend Guard or SourceGuardian especially for PHP source code. What topics should I look after about this matter ? thx for helping