PHP is used for server-side programming which will interact with databases to retrieve information, storing, email sending, and provides content to HTML pages to display on the screen. HTML is used for specifying colors, text formatting, aligning, etc. PHP is easy to learn but not as much as HTML.

php is displayed upon entering http://www.example.com. To view index. html, people must go directly to the page, for example by entering http://www.example.com/index.html.

While HTML allows for static content presentation, PHP enables developers to handle user input and process data dynamically. With PHP, developers can create interactive forms, perform server-side validations, and process user-submitted information.

At its most basic, index. php is the first file that will show up when a browser goes to the site or a directory. Beyond that, it can be used for whatever the site/project dictates. For more complicated sites or frameworks, it acts as the "boot" script.