PHP stands for Hypertext Preprocessor. It is server side scripting language which are used for Web development and can be embedded into HTML. PHP can be used to create dynamic and interactive web pages. PHP is widely used, because it's powerfull and also free.
PHP works almost every operating system, Linux, Unix, Windows, etc and can be used with different servers. Because PHP is free, so one of the most used servers with PHP are Apache, which are also free to download.