PHP (Hypertext Preprocessor) is a server-side programming language primarily used for back-end development. It is used to build dynamic web sites, web applications, and APIs. PHP code is executed on the server and generates HTML, CSS, and JavaScript that is sent to the client’s browser to be rendered.

PHP is commonly used in combination with a web server like Apache, and a database management system like MySQL, to create dynamic websites. It can be integrated with various front-end technologies like HTML, CSS and JavaScript to build a full-featured web application.

It is also used in many Content Management Systems (CMS) such as WordPress, Joomla, and Drupal, which are widely used for building websites and blogs.

In summary, PHP is primarily used for back-end development, but it can also be integrated with front-end technologies to build a full-featured web application.