From its original design as a way to create dynamic web pages, PHP has evolved into a language (and supporting environment) that is particularly well suited to rapidly developing web-scale applications. Examples of these include Facebook and Wikipedia.
Unfortunately, its genesis prevented it from being taken seriously as an enterprise-grade language and environment, especially when compared to the incumbent 800 pound gorilla, Java.
PHP is great even if you don't use Zend framework or others. The best thing is that you don't need any development environment, just write code and run your script on a php enabled web host. No compilation no license costs just open source.
It will be interesting the next years how popular PHP becomes on windows...