On PHPMaster.com today there's a new post from Abdullah Abouzekry showing you how to create QR codes (like these) with the help of a PHP library, PHP QR Code.

Used widely in various recent applications, QR Codes can be seen on cola cans, business cards, in sushi bars, and in museums. QR Code is a 2-dimensional barcode specification that was invented in Japan. [...] In this article I’ll show you how you can easily generate QR Codes from within your PHP application and share some ideas on how and when to use them, We’ll be using PHP QR Code, a library written in PHP for generating QR Codes and which doesn’t require any dependencies beyond the standard GD2 graphics extension for creating images.

Using the library makes things pretty easy - all you need to do is call the correct method related to the image type (like "png()" or "jpg()") for your desired output with the string you want to encode. You can also define things like the level of error correction of the result - related to how much data the image can store - and size.