A php file can contain the same as an html file but it also contains php code which is parsed by the server. So basically you take the html file you are using with all it’s contents and links to css and/or js files, cut out the parts you want to use as includes and do as it says in the tutorial. Add the php includes code and then save it out as “whatever.php”.
The css code should be in a separate file and linked to from the “head” section of your file so there is no need for you to use php include for it. You should not be using css inline unless it is a one off change on one page only. This is how you link to your css file.
All your css goes in this “main” file which is in a folder called “css”.