A QR Code is a matrix code (or two-dimensional bar code) created by Japanese corporation Denso-Wave in 1994. The "QR" is derived from "Quick Response", as the creator intended the code to allow its contents to be decoded at high speed.
Here is the QR code generator PHP class using Google Chart API the PHP source code can be downloaded from the author website here
Google Chart API
The Google chart API has so many useful stuff and one of them is the QR generator, here is an easy way to generate a desired QR Code;