How to encrypt a PHP file for free?

I have created a plugin for WordPress although I would like to encrypt it to stop people from viewing the source code. I've heard of Zend Guard and IonCube although is there a free alternative to these?

Looks like the things you choose are not working this way:

• Wordpress is open source
• PHP is not compiled
• any obfuscation can be decoded

If you don't want to share the source, provide the code as a service. You may require the user to enter API key etc.

• Execute the code on your server and send response (e.g. JSON or XML) to the Wordpress part.
• Allow only the users who paid for the service to access the service on your server
• Authenticate the users by selling and then checking the API key

See: SaaS