Actually, it's not that hard. I just made a template in which I added the code in each of the parts I wanted to put an html.
For example, once you've made a table, you include the code in the superior cell and point to your header document (a banner or just a title header), while the inferior cell will link to the main page. Then, you save the document with the .php extension instead of the .html one, and it's done!
Of course, you'll have to install a PHP program such as easy PHP in order to preview the pages, since when you open the .php file in the browser it doesn't read it.
It seems difficult, but once you grasp the concepts you'll be able to migrate the whole site in a few hours

Oh, as for the advantage... basically, when you want to change the layer, for example, as I did, you just have to worry about the design. The content pages are in html and won't be affected, so there are less broken links problems.
i don't know, it seems easier to me now. The only problem is that you have to make one .php file for each html subpage, but it's just cut, paste, change the page title and target url and that's all.