But how do I add link to the header what opens a .html file into the body (where usually the product list is)? Like a webpage or portal. Or I have to use layers or pop-up windows?
As you see, the link is "aboutUs", the file name is "about". See? The program automatically cuts the word where it finds the first uppercase letter. Like findUs -> find.php, johnDoe -> john.php. Got it? That's all. Everithing what you add to the $content variable will be shown where usually the product descriptions and pictures are, in the middle.
Take care with the apostrophes ( ' ) and PHP variables.