I want to ask you something about
<link rel='stylesheet' href='style/style1.css' />
href is not a file path, it's a URL, you use it to direct to another page.
include in PHP actually includes a separate file at that point in the page. Let's say you're building a php flat file site. Your header and footer are always the same but the pages change. You could include the header and footer as separate files instead of typing their code into each page.
<?php include 'header.php'; //the rest of your page code goes here. include 'footer.php';