Zend Optimizer
Zend Optimizer is an instrument, that is needed to run files encrypted with Zend Guard - a popular application that is used to encrypt PHP files with the purpose to protect them from tampering with the program code or reverse engineering. Zend Guard is used by a lot of organizations that create paid script applications, so when you get such an app for your website, you will probably need Zend Optimizer to be available on the server where you will host it. If you are a programmer, you can also use Zend Guard to protect your program code and make sure that your website visitors or customers won't be able to change it in any way. Sites that use Zend Optimizer generally perform better as their PHP code is precompiled, so it's already optimized and will be executed a lot quicker.