PHP is a programming language first released in 1995 intended for personal website development. Today it has the largest market share among similarly purposed languages. From a tool for simple web scripts, PHP has grown to a fully featured general purpose programming language.
PHP's audience is tremendously large, as its interpreter is installed on almost every webserver out there. The language has been used to solve thousands real-world problems, large and small.
Historically, one could create a PHP-based website in seconds without installing complex development tools, compiling the code or having to learn much at all. There are numerous lessons across the web on "how to create a PHP website in an hour". However, PHP is not limited to just that and can be used in many other situations.
Wordpress, Joomla and even Facebook were all written in PHP.