Zend Optimizer is a software instrument, that is required to run files encrypted with Zend Guard - a widespread application that is employed to encrypt PHP 4 and PHP 5 files with the objective to protect them from tampering with the program code or reverse engineering. Zend Guard is used by lots of organizations that develop paid script applications, so when you buy such an app for your website, you will probably need Zend Optimizer to be present on the server where you'll host it. If you are a programmer, you may also use Zend Guard to protect your code and ensure that your website visitors or clients won't be able to alter it in whatever way. Sites which use Zend Optimizer generally perform better because their PHP code is precompiled, thus it is already optimized and will be executed much more rapidly.