PHP is free software that generates dynamically HTML, CSS and JavaScript code.

It runs on the server side which means that it runs before any code like HTML, CSS or JavaScript will be read by your browser and presented to you.

The code that you see on your browser is HTML, CSS, JavaScript and XML.

With PHP you can manipulate data pulled out from a database and the (HTML, CSS, JavaScript or XML) code to create a desired outcome to display to users on their browsers. PHP uses many databases. The most popular one is MySQLi.