learning PHP

PHP, which stands for "PHP: Hypertext Preprocessor" is a powerful server-side scripting language that is especially suited for Web development and can be embedded into HTML. Its syntax draws upon C, Java, and Perl, and is easy to learn.

The main goal of the language is to allow web developers to write dynamically generated web pages quickly, but you can do much more with PHP.
PHP is often used together with Apache (web server) on various operating systems combine with Mysql (database).

If you are just learning PHP, XAMPP is an easy to install Apache distribution containing MySQL, PHP and Perl. XAMPP is really very easy to install and to use - just download, extract and start.
It is available for linux, windows, Mac OS X and Solaris.

XAMPP is a compilation of free software (comparable to a Linux distribution), it's free of charge and it's free to copy under the terms of the GNU GENERAL PUBLIC LICENSE. But it is only the compilation of XAMPP that is published under GPL. Please check every single license of the contained products to get an overview of what is, and what isn't, allowed.

just keep in mind that these installation packages are for development use, and are not built for a production enviroment, the preformance and security of these tools is not setup for use in a public website.