Is a PHP source code library that allows you to write PHP web applications. It includes a MVC framework, handles dispatching of requests and contains useful utilities that most PHP programmers need.
It does not depend upon a specific web server and can run under any web server running a compatible version of PHP.
Is a repackaged version of Apache. It contains a control panel that allows you to configure PHP settings (those found in the
php.ini file), and also enable/disable extensions for PHP.
Zend Server has a free community version and a commercial version. With all of it's features enabled, it allows a PHP application to support Zend's caching, code compiling and obfuscation.
Now I've run regular Apache with PHP, and Zend Server on the same machines. In most cases I've found Zend Server to perform better (despite the fact that it's also using Apache). Out of the box Zend Server comes configured better to run PHP. If you are an expert at installing PHP on Apache then you might be able to reproduce the results, but for the average programmer I recommend Zend Server if it's an option.