PHP is a programming and scripting language to create dynamic interactive websites. WordPress is written using PHP as the scripting language. Just like WordPress, PHP is also an Open Source.
PHP is a server side programming language. When a user requests a web page that contains PHP code, the code is processed by the PHP module installed on that web server. The PHP pre-processor then generates HTML output to be displayed on the user’s browser screen.
(Referenced from “What is: PHP” )