php is used for server-side development of web apps, making it possible to administrate your website using only browser.

Formal description:

PHP - a scripting language used for general-purpose programming and extensively used for web application development. Currently PHP is supported by the overwhelming majority of web hosting providers, and is one of the leading programming languages used to create dynamic websites.

In other words:

PHP - a programming language that allows you to process information on your website as a portal administrator or a portal visitor, as well as build, customize, or extend on the basis of this information.

What is it used for?

This programming language is the basis for most websites. It is the use PHP, together with the database administration panel which allows you to update a website without programming skills. In addition, thanks to the use of PHP, you can incorporate custom forms within your site, create custom reporting and implement user authentication systems.
By combining PHP, with additional libraries responsible for sending new information to the server, and other multimedia files (images, documents, video, etc.), a dynamic website is born.