php仿QQ验证码的实例分析_php技巧

有需要的朋友，参考下吧。
代码如下:
/**
* 仿QQ验证码
*/
//Session保存路径
$sessSavePath = dirname(__FILE__)."/../data/sessions/";
if(is_writeable($sessSavePath) && is_readable($sessSavePath)){ session_save_path($sessSavePath); }
session_start();
//获取随机字符
$rndstring = '';
for($i=0; $i<4; $i++) $rndstring .= chr(mt_rand(65,90));
$img_height=45; //先定义图片的长、宽
$img_width=10;
//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
$im = imagecreate(46,20);
ImageColorAllocate($im, 240,243,248);
//干扰线
$lineColor1 = ImageColorAllocate($im, mt_rand(174,218),mt_rand(190,225),mt_rand(217,237));
for($j=1;$j<=2;$j=$j+3)
{
imageline($im,0,$j+mt_rand(1,15),48,$j+mt_rand(1,15),$lineColor1);
}
//输出文字
$fontColor = ImageColorAllocate($im, mt_rand(0,150),mt_rand(0,150),mt_rand(0,150));
for($i=0;$i<$rndcodelen;$i++)
{
$bc = mt_rand(0,1);
$rndstring[$i] = strtoupper($rndstring[$i]);
imagestring($im,mt_rand(3,5),$i*$img_height/4+mt_rand(1,5),mt_rand(1,$img_width/2), $rndstring[$i], $fontColor);
}
header("Pragma:no-cache"r"n");
header("Cache-Control:no-cache"r"n");
header("Expires:0"r"n");
//输出特定类型的图片格式，优先级为 gif -> jpg ->png
if(function_exists("imagepng"))
{
header("content-type:image/png"r"n");
imagepng($im);
}
else
{
header("content-type:image/jpeg"r"n");
imagejpeg($im);
}
ImageDestroy($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/png"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();
}
?>