PHP is a scripting language that can be used to create dynamic websites or build standalone interpreted applications. The package includes everything needed to run and create PHP scripts. That includes the basic libraries and runtime interpreter or preprocessor. The language was very popular some time ago because it was simple to understand and easy to insert into web pages. It has since declined greatly in popularity because of more modern languages like Python and inherent problems with PHP itself. The download package is for PHP developers or people who want to run a PHP script. Installation can be tricky if you are not familiar with computers.