Script code and HTML formatting are interpreted and presented by the web browser. This is called client-side code. The processes involved in retrieving the information requested by the client, session tracking, and most of the application's logic are executed in the server through the server-side code, written in languages such as PHP, ASP.NET, Java, Python, Ruby, and JSP. This code produces an output that can then be formatted using HTML. When you see a URL ending with a .php extension, it indicates that the page may contain PHP code. It then must run through the server's PHP engine, which allows dynamic content to be generated when the web page is loaded.