dedecms验证码的代码解析

<?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();
}
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();
}
?>