PHP is a scripting language designed to fill the gap between SSI (Server Side Includes) and Perl, intended for the Web environment. Its principal application is the implementation of Web pages having dynamic content. PHP has gained quite a following in recent times, and it is one of the frontrunners in the Open Source software movement. Its popularity derives from its C-like syntax, and its simplicity. The newest version of PHP is 7.0 and it is heavily recommended to always use the newest version for better security, performance and of course features.
If you've been to a website that prompts you to login, you've probably encountered a server-side scripting language. Due to its market saturation, this means you've probably come across PHP. PHP is even used to run sites such as Wikibooks. PHP was designed by Rasmus Lerdorf to display his resume online and to collect data from his visitors.
Basically, PHP allows a static webpage to become dynamic. "PHP" is an acronym that stands for "PHP: Hypertext Preprocessor". The word "Preprocessor" means that PHP makes changes before the HTML page is created. This enables developers to create powerful applications that can publish a blog, remotely control hardware, or run a powerful website such as Facebook or Wikipedia. Of course, to accomplish something such as this, you need a database application such as MySQL.
Before you embark on the wonderful journey of Server Side Processing, it is recommended that you have a basic understanding of the HyperText Markup Language (HTML). But PHP can also be used to build GUI-driven applications for example by using PHP-GTK.