dedecms验证码的代码解析

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 $bordercolor = ImageColorAllocate($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], $fontColor); } header("Pragma:no-cache/r/n"); header("Cache-Control:no-cache/r/n"); header("Expires:0/r/n"); //输出特定类型的图片格式，优先级为 gif -> jpg ->png if(function_exists("imagejpeg")) { header("content-type:image/jpeg/r/n"); imagejpeg($im); } else { header("content-type:image/png/r/n"); imagepng($im); } ImageDestroy($im); exit(); }//by else { //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(); } ?>