PHP: Hypertext Preprocessor is an open source server side programming language extensively used for web scripts and to process data passed via the Common Gateway Interface from HTML forms etc. PHP can be written as scripts that reside on the server and may produce HTML output that downloads to the web browser. Alternatively, PHP can be embedded within HTML pages that are then saved with a .php file extension. The PHP sections of the page are then parsed by the PHP engine on the server and the PHP code stripped out before the page is downloaded to the web browser.
The name is a bit of a programming joke (if there is such a thing) since it’s a recursive acronym i.e. the P in PHP stands for PHP.