How do i use PHP in a HTML File?

im a newbie in web development and im learning it by myself, so please have mercy if the question sounds stupid. :)

Now im a little bit stucked and i dont know how to solve the problem.

Im starting to learn PHP now, i have installed XAMPP. I moved all the files to the /htdoc directory of XAMPP. I renamed the index.html data to index.php to make it work. But now the Live Preview of Brackets is just showing me the PHP part but not the HTML, CSS and JavaScript elements which i defined in the same file. When i change the ending of the file to .html, it shows me the elements but not the PHP Part.

I dont really know how to handle the problem. Do i have to define the PHP part in a external .php file and link it to the .html file? I dont really find a answer in the internet.

Thank you in advance.