PHP menu on side rather than "stacking" on top?

Here’s an example of what I’m talking about :

See how the menu is on the left side rather than along the top…I’m new to PHP and haven’t been able to find any info on this. All I know how to do is “stack” header.php, main.php, and footer.php “on top” of eachother. How can I “stack” these side by side?

Here’s the code I use to “stack” them:

<?php include ("header.php"); ?>
<?php include ("main.php"); ?>
<?php include ("footer.php"); ?>

Any help appreciated!