An example of a SSI in PHP.
$title = "Index";
Now we put our contents in here!!
Then in header we could add
<link rel="stylesheet" href="styles/main.css" />
<title>Site Name - <?php echo "$title"; ?></title>
Which would make a page that has a different title on each page given by the $title variable. Also, all you would need to do to change a layout or add a link to your pages would be to edit the footer and header includes.