php生成验证码详细教程

$numimage = imagecreate($imagewidth,$imageheight);
imagecolorallocate($numimage,240,240,240);
for($i=0;$i<strlen($num);$i++){
$x = mt_rand(1,8)+$imagewidth*$i/4;
$y = mt_rand(1,$imageheight/4);
$color=imagecolorallocate($numimage,mt_rand(0,150),mt_rand(0,150),mt_rand(0,150));
imagestring($numimage,5,$x,$y,$num[$i],$color);
}

for($i=0;$i<200;$i++){
$randcolor=imagecolorallocate($numimage,rand(200,255),rand(200,255),rand(200,255));
imagesetpixel($numimage,rand()%70,rand()%20,$randcolor);
}
imagepng($numimage);
imagedestroy($numimage);
?>

   //生成验证码图片

    session_start();

        header("content-type: image/png"); 

  srand((double)microtime()*1000000); 

  $roundnum=rand(1000,9999);

  //把随机数存入session以便以后用

   $_session["sessionround"]=$roundnum;

        $im = imagecreate(58,28);

        $red = imagecolorallocate($im, 255,0,0);

        $blue = imagecolorallocate($im, 0,255,0);

 //局域填充，相当于背景

        imagefill($im,68,30,$red);

   //将四位整数验证码绘入图片

        imagestring($im, 5, 10, 8, $roundnum, $blue);

        for($i=0;$i<50;$i++)   //加入干扰象素

        {

                imagesetpixel($im, rand()%70 , rand()%30 , $black);

        }

        imagepng($im);

        imagedestroy($im);

?>

     <input name="validt" type="text" size="10" maxlength="4"/></td><td colspan="3" width="80%">

     <img src="/www.111cn.net/validitpicture.php" name="validitpic" align="absmiddle"  /></td>