Lots of licensed and some free script-driven applications have encrypted code, which isn't human readable. The idea behind this is to prevent the reverse engineering as well as the unauthorized usage of such applications. One of the most popular encryption tools used for this particular purpose is called Zend Guard and it is widely used because it can be used to modify all PHP 4 or PHP 5 code. The only method for the encoded files to run effectively on a web server afterwards is if a second tool called Zend Optimizer is present. If you want to employ any type of paid web software requiring Zend Optimizer, you should ensure that it is installed on the server where you will host your site. In addition, sites that need the instrument generally perform better because their program code is already precompiled and optimized, meaning that it's executed faster.