PHP is a programming language that can do all sorts of things: evaluate form data sent from a browser, build custom web content to serve the browser, talk to a database, and even send and receive cookies (little packets of data that your browser uses to remember things, like if you're logged in to Codecademy).
Check out the code in the editor. Looks familiar, doesn't it? That's because a lot of it is regular old HTML! The PHP code is written in the
?> . See how it generates numbers, creates lists, and adds text directly to your webpage?