英文同数字
<?php
Header("Content-type:image/png");
//定义header，声明图片文件，最好是png，无版权之扰;
//生成新的四位整数验证码
session_start();//开启session;
$authnum_session='';
$str='abcdefghijkmnpqrstuvwxyz1234567890';
//定义用来显示在图片上的数字和字母;
$l=strlen($str);//得到字串的长度;
//循环随机抽取四位前面定义的字母和数字;
for($i=1;$i<=4;$i++)
{
$num=rand(0,$l-1);
//每次随机抽取一位数字;从第一个字到该字串最大长度,
//减1是因为截取字符是从0开始起算;这样34字符任意都有可能排在其中;
$authnum_session.=$str[$num];
//将通过数字得来的字符连起来一共是四位;
}
session_register("authnum_session");
//用session来做验证也不错;注册session,名称为authnum_session,
//其它页面只要包含了该图片
//即可以通过$_SESSION["authnum_session"]来调用

//生成验证码图片，
srand((double)microtime()*1000000);
$im=imagecreate(50,20);//图片宽与高;
//主要用到黑白灰三种色;
$black=ImageColorAllocate($im,0,0,0);
$white=ImageColorAllocate($im,255,255,255);
$gray=ImageColorAllocate($im,200,200,200);
//将四位整数验证码绘入图片
imagefill($im,68,30,$gray);
//如不用干扰线，注释就行了;
$li=ImageColorAllocate($im,220,220,220);
for($i=0;$i<3;$i++)
{//加入3条干扰线;也可以不要;视情况而定，因为可能影响用户输入;
imageline($im,rand(0,30),rand(0,21),rand(20,40),rand(0,21),$li);
}
//字符在图片的位置;
imagestring($im,5,8,2,$authnum_session,$white);
for($i=0;$i<90;$i++)
{//加入干扰象素
imagesetpixel($im,rand()%70,rand()%30,$gray);
}
ImagePNG($im);
ImageDestroy($im);
?>

中文

<?php
/*
*文件:code.php
*作用:验证码生成
*/
session_start();
//设置content-type
header("Content-type:image/png");
//创建图片
$im=imagecreatetruecolor(120,30);
$ChineseChar=array("人","出","来","友","学","孝","仁","义","礼","廉","忠","国","中","易","白","者","火","土","金","木","雷","风","龙","虎","天","地",
"生","晕","菜","鸟","田","三","百","钱","福","爱","情","兽","虫","鱼","九","网","新","度","哎","唉","啊","哦","仪","老","少","日",
"月","星");
//创建颜色
$fontcolor=imagecolorallocate($im,255,255,255);
$bg=imagecolorallocate($im,0,0,0);

//设置文字
for($i=0;$i<4;$i++)$text.=$ChineseChar[(array_rand($ChineseChar))];

$_SESSION['code']=$text;

//设置字体[url]http://.cn/downlist/s_12_3.html[/url]有_GBK系列的字体下载,一般GD库都支持的!
$font='gbk.ttf';

//添加文字
imagettftext($im,18,0,11,21,$fontcolor,$font,iconv("GB2312","UTF-8",$text));

//输出图片
imagepng($im);
imagedestroy($im);
?>