PHP is a widely-used general-purpose scripting language that is especially suited for web development and can be embedded into HTML code. It generally runs on a web server, taking PHP code as its input and creating web pages as output. It can be deployed on most web servers and on almost every operating system and platform free of charge.
A bit less technicaly speaking, most of the dynamic websites you open are generated by a program that was written in PHP.
As PHP itself is open source and is widely used for web projects, most of the open source web applications are built with PHP.