PHP is a scripting language that runs on the server and which allows dynamic generation of web pages. PHP to connect to a database to generate the content. PHP allows you to edit data that are pre-entered via a form. PHP allows you to add an amazing amount of functionality to your website.
PHP is just "part of the puzzle" in designing the website. PHP works in conjunction with HTML to display data on a web page. HTML / CSS is what you see. PHP is the logic that determines what appears on the page.