PHP is a general-purpose server side scripting language used to create dynamic web applications. The original expansion of the acronym was personal home page. PHP is a scripting language that is very C-like, although like all other scripting languages, it is not compiled. It is a very simple language to learn, but is extremely powerful.
PHP is usually used to add a functionality that HTML alone can't handle. For example, with PHP you can collect user information as well as interact with MySQL databases. It can also be used to perform complex mathematics as well as determine what day of the week a particular date falls on, either past or future.