Zend Optimizer is a software tool, that's required to run files encrypted with Zend Guard - a popular app that is used to encrypt PHP 4, PHP 5, PHP 7 and PHP 8 files with the objective to protect them from tampering with the program code or reverse engineering. Zend Guard is used by a lot of companies that develop paid script applications, so in case you purchase such an app for your website, you'll most likely need Zend Optimizer to be present on the server where you'll host it. In case you're a programmer, you can also use Zend Guard to protect your program code and ensure that your website visitors or clients won't be able to change it in any way. Sites that use Zend Optimizer usually perform much better owing to the fact that their PHP code is precompiled, therefore it is already optimized and will be executed much more rapidly.