Split php and html

Question

hi,
I'm currently looking for a way or lesson on how to split PHP (code) and html (Theme) into two different pages .php.
the aim is so that I can easily modify the theme and maybe enable multiple themes (the theme choice will be imposed be me)
I was planning on having the index.php call the load page that will load the theme and code
i know that this is possible (seen it done) but I can't figure out how
could anyone help me ?
please, thanks in advance

Answer 1

unfortunately I wanted to put the theme in a different folder