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！');
}