phpGD库-验证码

$str = '1234567890qwertyuiopasdfghjklzxcvbnmZXCVBNMASDFGHJKLQWERTYUIOP';
        switch ($type) {
            case 1 :
                //表示验证码是纯数字
                $start = 0;
                $end = 9;
                break;
            case 2 :
                //表示验证码是纯字母
                $start = 10;
                $end = 61;
                break;
            case 3 :
                //数字和字母
                $start = 0;
                $end = 61;
                break;
        }
        //生成随机的字符串
        $newstr = '';
        for($i = 0; $i < $length; $i++){
            $sj = mt_rand($start,$end);
            $newstr .= substr($str, $sj, 1);//***********
        }

//echo mb_substr($str ,0,4,'utf-8');截取4个
//echo mb_strlen($str,'utf-8');// 获取长度x,,原始3x
$str = '们以我到他会作时要动国产的一是工就年阶义发成部民可出能方进在了不和有大这主中人上为来分生对于学下级地个用同行面说种过命度革而多子后自肩腾枯抛轨堂拌爸循诱祝励肯酒绳穷塘燥泡袋朗喂铝软渠颗惯贸粪综墙趋彼届墨碍启逆卸航衣孙龄岭骗休借';
$newstr = '';
for($i = 0; $i < 4; $i++){
    $start = mt_rand(0,mb_strlen($str,'utf-8')-1);//每次减去1个
    $newstr .= mb_substr($str,$start,1,'utf-8');
}

echo $newstr;