PHP is a server-side scripting language designed for web development but also used as a general-purpose programming language. As of January 2013, PHP was installed on more than 240 million websites (39% of those sampled) and 2.1 million web servers. While PHP originally stood for Personal Home Page, it now stands for PHP:
PHP is open source, meaning that everyone has free access to the source code and can use it for their own development purposes. A key advantage of the open source model is that you avoid vendor lock-in.
There has been an enormous increase in PHP adoption and many large web applications have been developed in PHP