Web Programming:

A file extension is a way to indicate to the operating system what the file format is. With the correct file extension, the operating system knows what to do when the user opens the file. You know HTML and all files you create have the html file extension. PHP and all files you create have the php file extension.

Whats the difference between HTML and PHP file extensions? When would you use HTML file extension? When would you use PHP file extensions? Explain why.