Create Google AdSense shortcode for in WordPress

AdSense-in-WordPress.php

<?php
/*
 * enqueue adsbygoogle.js in the footer & 
 * create a WordPress shortcode with your Google AdSense code
 * follow me on Twitter: @Jan_Reilink
 */

function saotn_loadAdsByGoogleJs() {
  wp_register_script( 'google-adsense', '//pagead2.googlesyndication.com/pagead/js/adsbygoogle.js', '', '', true );
  wp_enqueue_script( 'google-adsense' );
}
add_action( 'wp_enqueue_scripts', 'saotn_loadAdsByGoogleJs' );

function saotn_showAdSense() {
  $output = '<ins class="adsbygoogle"
    style="background-color: transparent;
      text-decoration: none;
      display:inline-block;
      width:336px;
      height:280px"
    data-ad-client="ca-pub-xxxxxx"
    data-ad-slot="xxxxxx"></ins>
  <script>
  (adsbygoogle = window.adsbygoogle || []).push({});
  </script>';
  
  return $output;
}
add_shortcode( 'showAdSense', 'saotn_showAdSense' );

/*
 * Usage: add [showAdSense] in your WordPress posts 
 * where you want your ad to appear
 */

A Dutch article with more information and usage:
https://www.itfaq.nl/google-adsense-wordpress-shortcode/