PHP is a general-purpose scripting language: a programming language whose code is read and run by an interpreter when it is needed, without a separate compile step. The name is a recursive acronym for "PHP: Hypertext Preprocessor". The PHP Group and its contributors develop it, and it is made especially for web development.
PHP code can sit inside an HTML page, between the tags <?php and ?>. The code runs on the server, and only the HTML it generates goes to the browser, so a visitor never sees the code. The interpreter runs as a module of a web server such as Apache, or as a CGI processor (a separate program that the web server calls), for example through FastCGI behind nginx. It also runs command-line scripts, such as scheduled jobs. PHP runs on Linux, other Unix systems, Windows and macOS. Extensions connect it to databases such as MySQL, also through PDO (a common database interface), and produce images, PDF files, JSON and XML, or send emails.
PHP is free and open source under the Modified BSD License, also called the PHP License version 4, a permissive license.