loading content thru PHP to simplify or is there a way to load it in a div?

I have my header/css and part of the body including the <div tag for the cell the content is in as one file and then the footer with the and and segment as another file and then have the following code:
How do I make it so it will load content per a PHP page with the content in it… it’s all supposed to go in one div on the page and then the header and footer (the parts of the page that stay the same which is everything except that div) saved as separate files to minimize loading time…

<!DOCTYPE html>
<html lang="en">
    <?php

$PageTitle="Havent // Heavent Made // Hellt Mindedt // Manualt";

function customPageHeader(){?>
  <!--Arbitrary HTML Tags-->
<?php }

include_once('file:///home/chronos/u-57608984ca0e6dfae1c20c6c0ed79e71e9626f08/MyFiles/Downloads/header.php');

include_once("content");

include_once('file:///home/chronos/u-57608984ca0e6dfae1c20c6c0ed79e71e9626f08/MyFiles/Downloads/footer.php');
?>