写了一个PHP随机问答验证码，安全性个人认为还是不错的。

<?php
//$i为要写在画板的位置，$num为要写的文字
function rand4string($i,$num){
global $image_width,$image_height,$img;
$font=mt_rand(3,5);
$x=mt_rand(1,8)+$image_width*$i/6;
$y=mt_rand(1,$image_height/4);
$color=imagecolorallocate($img,mt_rand(0,100),mt_rand(0,150),mt_rand(0,200));
imagestring($img,$font,$x,$y,$num,$color);
}
session_start();
header("content-type:image/png");
$image_width=80;
$image_height=18;
$img = imagecreate($image_width,$image_height);
imagecolorallocate($img,255,255,255);
$random4symbol = array("+","-","x");	//三个符号进行循环
$result = 0;
$symbol = $random4symbol[mt_rand(0,count($random4symbol)-1)];
$num1 = mt_rand(1,9);
rand4string(0,$num1);
$num2 = mt_rand(1,9);
switch ($symbol){
case "+":
$result = $num1 + $num2;
rand4string(1,"+");
break;
case "-":
$result = $num1 - $num2;
rand4string(1,"-");
break;
case "x":
$result = $num1 * $num2;
rand4string(1,"x");
break;
}
rand4string(2,$num2);
rand4string(3,"=");
rand4string(4,"?");
$_SESSION['code'] = $result;
imagepng($img);
imagedestroy($img);
?>