dedecms验证码的代码解析。本文介绍下，织梦dedecms中验证码代码的实现方法，有需要的朋友，可以参考学习下。有关dedecms验证码的代码，大家注意看其中的注释，以帮助理解哦。代码如下：
<?php$rndstring = '';for($i=0; $i<4; $i++) $rndstring .= chr(mt_rand(65,90));//如果支持GD，则绘图if(function_exists("imagecreate")){//Firefox部份情况会多次请求的问题，5秒内刷新页面将不改变session$ntime = time();if(empty($_SESSION['dd_ckstr_last']) || empty($_SESSION['dd_ckstr']) || ($ntime - $_SESSION['dd_ckstr_last'] > 5)){$_SESSION['dd_ckstr'] = strtolower($rndstring);$_SESSION['dd_ckstr_last'] = $ntime;}$rndstring = $_SESSION['dd_ckstr'];$rndcodelen = strlen($rndstring);//创建图片，并设置背景色$im = imagecreate(50,20);ImageColorAllocate($im, 255,255,255);//背景线$lineColor1 = ImageColorAllocate($im,240,220,180);$lineColor2 = ImageColorAllocate($im,250,250,170);for($j=3;$j<=16;$j=$j+3){imageline($im,2,$j,48,$j,$lineColor1);}for($j=2;$j<52;$j=$j+(mt_rand(3,6))){imageline($im,$j,2,$j-6,18,$lineColor2);}//画边框www.genban.org$bordercolor = ImageColorAllocate($im, 0x99,0x99,0x99);imagerectangle($im, 0, 0, 49, 19, $bordercolor);//输出文字$fontColor = ImageColorAllocate($im, 48,61,50);for($i=0;$i<$rndcodelen;$i++){$bc = mt_rand(0,1);$rndstring[$i] = strtoupper($rndstring[$i]);imagestring($im, 5, $i*10+6, mt_rand(2,4), $rndstring[$i], $fontColor);}header("Pragma:no-cache/r/n");header("Cache-Control:no-cache/r/n");header("Expires:0/r/n");//输出特定类型的图片格式，优先级为 gif -> jpg ->pngif(function_exists("imagejpeg")){header("content-type:image/jpeg/r/n");imagejpeg($im);}else{header("content-type:image/png/r/n");imagepng($im);}ImageDestroy($im);exit();}//by www.jbxue.comelse{//不支持GD，只输出字母 ABCD$_SESSION['dd_ckstr'] = "abcd";$_SESSION['dd_ckstr_last'] = '';header("content-type:image/jpeg/r/n");header("Pragma:no-cache/r/n");header("Cache-Control:no-cache/r/n");header("Expires:0/r/n");$fp = fopen("data/vdcode.jpg","r");echo fread($fp,filesize("data/vdcode.jpg"));fclose($fp);exit();}?>
