PHP is a programming language that is often used to build web applications and content management systems (CMS). Many CMSs are built using PHP, including popular platforms such as WordPress, Drupal, and Joomla. PHP is a server-side language, which means that it is executed on the server and the results are sent to the client's web browser. PHP is often used in conjunction with a database to store and retrieve data, such as user information and website content.