Generating ads after 1st post from external file

Hello Everyone. I’m currently using the code I found at https://www.tamba2.org.uk/wordpress/adsense/ to post ads within a certain number of post.

I have statics ads… which is basically an image file on my server that links to a web address. I applied this method to the following theme file: index.php, archive.php, and single.php.

This is a small task but I would like to make my work load a little lighter. Since all these pages are using the same images anyways, is there some way I can you may an include or something to the ad code from one central file whether it be .html, .txt, or .php?

Here are the codes that I used from the website.
The variable

<?php
$postnum = 1;
$showadsense1 = 1;
?>

The output

<?php if ($postnum == $showadsense1) {
echo '
AD CODE HERE
';
} ?>

<?php $postnum++; ?>

I’m thinking something like an include can be added somewhere in the second portion of the code. I’m not a php person at all. Any advice would be greatly appreaciated.