QR code stands for quick response code, created to be scanned in with your mobile device. They are starting to become more and more popular the more mobile devices are being used.
If you want to generate QR codes for your blog posts, add the following code into your current theme’s single.php file. Add this line of code in the post where your want to show the QR code.
<img src="http://api.qrserver.com/v1/create-qr-code/?size=120x120&data=<?php the_permalink(); ?>" alt="QR - <?php the_title(); ?>"/>
You can change the size of QR image form size=120×120 to your desired size. And add this code in theme’s page.php file if you want QR code to be displayed in the pages.