I apologize if this appears to be a silly question. I am an old newbie but decided to understand all of this.
I duplicated my home page which is .PHP file so now I have my second page which is equally a .php. so my question is: is it ok to have all the pages of the site in .php ? Or will it be a problem when I publish? When I try to change for .html it always comes back to .php as soon I view another page
The reason it keeps reverting to .php when you change it to .html, navigate away, then navigate back, is that you have placed at least one stack in your page that is forcing the page to be .php to function correctly.
If you were to remove that stack or stacks from the page, you would be able to change the page to .html and have it stay like that, but it won’t make any difference to you if this is a new page your creating as you will not lose any already amassed inbound search engine links, because there are none yet.