QR Codes are quite popular online theses days. Do you want to generate QR codes for all of your blog posts? Here is a very simple way to do it.
Copy the following code and paste it into your single.php file, where you want the QR code to be displayed.
If you also want to provide a QR code on pages, paste the code into page.php as well.
<img src="https://api.qrserver.com/v1/create-qr-code/?size=100x100&data=<?php the_permalink(); ?>" alt="QR: <?php the_title(); ?>"/>
Thanks to Kevin Chard for the tip!