when to use index.php instead of index.html

Question

I am relatively new to php. There is a very basic thing that has been troubling me. I understand that php is used to make web sites dynamic. I also understand that php is on

Answer 1

For example, in my index.php or another main page I mainly use php because I use variables w/ them in the rest of my site:

Because I every time I link an image in my website so it doesn't break or something. Since I'm reusing the name all the time, I use .php to be able to have that service, because if I use the name, I can change it globally.

I think that simple pages like 404.html, aboutus.html, or ViewOneBlogPost.html could be an HTML page, you might not need any functionality/variables for that.