thinkphp中的验证码的实现

public function verify(){
        // 验证码
        import("@.Util.Image");
        Image::buildImageVerify(4,1,'png',40,20,'verify');
    }

 /**
     * 生成图像验证码
     * @static
     * @access public
     * @param string $length  位数
     * @param string $mode  类型
     * @param string $type 图像格式
     * @param string $width  宽度
     * @param string $height  高度
     * @return string
     */
    static function buildImageVerify($length=4, $mode=1, $type='png', $width=48, $height=22, $verifyName='verify') {
        import('ORG.Util.String');
        $randval = String::randString($length, $mode);
        session($verifyName, md5($randval));
        $width = ($length * 10 + 10) > $width ? $length * 10 + 10 : $width;
        if ($type != 'gif' && function_exists('imagecreatetruecolor')) {
            $im = imagecreatetruecolor($width, $height);
        } else {
            $im = imagecreate($width, $height);
        }
        $r = Array(225, 255, 255, 223);
        $g = Array(225, 236, 237, 255);
        $b = Array(225, 236, 166, 125);
        $key = mt_rand(0, 3);

        $backColor = imagecolorallocate($im, $r[$key], $g[$key], $b[$key]);    //背景色（随机）
        $borderColor = imagecolorallocate($im, 100, 100, 100);                    //边框色
        imagefilledrectangle($im, 0, 0, $width - 1, $height - 1, $backColor);
        imagerectangle($im, 0, 0, $width - 1, $height - 1, $borderColor);
        $stringColor = imagecolorallocate($im, mt_rand(0, 200), mt_rand(0, 120), mt_rand(0, 120));
        // 干扰
        for ($i = 0; $i < 10; $i++) {
            imagearc($im, mt_rand(-10, $width), mt_rand(-10, $height), mt_rand(30, 300), mt_rand(20, 200), 55, 44, $stringColor);
        }
        for ($i = 0; $i < 25; $i++) {
            imagesetpixel($im, mt_rand(0, $width), mt_rand(0, $height), $stringColor);
        }
        for ($i = 0; $i < $length; $i++) {
            imagestring($im, 5, $i * 10 + 5, mt_rand(1, 8), $randval{$i}, $stringColor);
        }
        Image::output($im, $type);
    }

<img id="verifyImg" src="{sh::U('Agent/Login/verify')}" onClick="changeVerify()" title="点击刷新验证码" /></div>

function changeVerify(){
 verifyURL = "{sh::U('Agent/Login/verify')}";
 $("#verifyImg").attr("src",verifyURL);
 return false;
}

if($_SESSION['verify'] != md5($_POST['verify'])) {
    $this->error('验证码错误！');
}