Protecting Code source.

Hello, I'd like to sell an application to 24$ US and I have to protect my source code. But I don't want to make my customer install anything on the server to be able to run my application. So the Zend encoder seems good for me: http://www.zend.com/store/products/zend-encoder.php "Or, make it easy for your customers to run Zend-encoded files by bundling your application with the Zend Optimizer freeware." The only trouble is the price (960$), it's a little expensive for the student I am :-/ Does anyone know any other less expensive way to protect my source code ? Thanks, Vincent.