QR码详细原理可以参考QR维基百科，中文版介绍比较少，英文版介绍很全面，推荐看英文版的。

　　Description

　　PHP QR Code is open source (LGPL) library for generating QR Code, 2-dimensional barcode. Based on libqrencode C library, provides API for creating QR Code barcode images (PNG, JPEG thanks to GD2). Implemented purely in PHP.

　　Features

　　•Supports QR Code versions (size) 1-40

　　•Numeric, Alphanumeric, 8-bit and Kanji encoding.

　　•Implemented purely in PHP, no external dependencies except GD2

　　•Exports to PNG, JPEG images, also exports as bit-table

　　•TCPDF 2-D barcode API integration

　　•Easy to configure

　　•Data cache for calculation speed-up

　　•Debug data dump, error logging, time benchmarking

　　•Provided merge tool helps deploy library as a one file

　　•new! API documentation

　　•new! Over 30 detailed examples

　　•100% Open Source, LGPL Licensed

　　Demo

　　项目地址：http://sourceforge.net/projects/phpqrcode/

　　Generate fixed size images

　　* phpqrcode本身未提供固定大小二维码的设置，请参考 http://sourceforge.net/p/phpqrcode/discussion/1111884/thread/f739531a/

 代码如下  

--- original/phpqrcode/qrimage.php       2010-08-18 07:06:24.000000000 -0700
+++ qrimage.php 2012-07-13 00:51:57.000000000 -0700
@@ -86,10 +86,15 @@
                 }
             }
-            $target_image =ImageCreate($imgW * $pixelPerPoint, $imgH * $pixelPerPoint);
-            ImageCopyResized($target_image, $base_image, 0, 0, 0, 0, $imgW * $pixelPerPoint, $imgH * $pixelPerPoint, $imgW, $imgH);
+
+            $targetW = (defined('IMAGE_WIDTH') ? IMAGE_WIDTH : $imgW * $pixelPerPoint );
+            $targetH = (defined('IMAGE_HEIGHT') ? IMAGE_HEIGHT : $imgH * $pixelPerPoint );
+
+            $target_image =ImageCreate($targetW, $targetH);
+
+            ImageCopyResized($target_image, $base_image, 0, 0, 0, 0, $targetW, $targetH, $imgW, $imgH);
             ImageDestroy($base_image);
             return $target_image;
         }
-    }
No newline at end of file
+    }

用法，设置生成的二维码宽高为 500px

define('IMAGE_WIDTH', 500);
define('IMAGE_HEIGHT', 500);
QRcode::png('http://www.111cn.net/)');