To create PHP files, you can switch the file extension to PHP in the Page Manager. For some of you this will be something new, and therefore we have provided a small exercise. The 'demo' above is just an image, because this site is hosted on the Coffeecup S-drive which is not able to handle php files. Also, the 'components' here are zip files, and they are best saved as templates.
'PHP includes' are a bit like the 'Symbols'. They work in much the same way, but they are created differently. And one advantage is, that the files will be smaller in bits and bytes because there will only be one header, or one footer, not one on every page.
While working with this in Site Designer, you will see what happens on the canvas, but in local browser preview the areas which are replaced using the PHP include tags will appear empty. Since PHP is a 'server side' language, the various parts of a web page will be transferred to the server as separate files, but when opened online in a browser, all the parts will be put together to make up a complete page.
The full explanation about how to create a page (or a whole site) using PHP includes has been written into the downloadable file. Therefore, saving it as a template really is the best advice.