PHP Includes and Server Side Includes (SSI) both allow you to insert a chunk of code into any page you want to by including a tag where that code goes. It could insert all the code for your header from a single file into each page on your site. It, for instance, keeps you from having to edit every page when you make a single change to that header. It looks to a browser as if it's code that is physically part of that page all along, and each page will load in the browser as if it's a new page. That means the header code will load again with each new page, just as if it were part of each new page (which it is).