Template engine with PHP

Our goal is to use static header (header.html) and footer (footer.html) while the content in between is changing (content1.html, content2.php, main.html). In the case where no page is selected the main page (main.html) will be displayed by default and if the name does not exist it will display the error page (error.html). Notice that the content page as we name it can be HTML or PHP.