public function entry_add($id = '') {
    $this->length='3';
    // 图片宽(px)
    $this->imageW || $this->imageW = $this->length*$this->fontSize*1.5 + $this->length*$this->fontSize/2; 
    // 图片高(px)
    $this->imageH || $this->imageH = $this->fontSize * 2.5;
    // 建立一幅 $this->imageW x $this->imageH 的图像
    $this->_image = imagecreate($this->imageW, $this->imageH); 
    // 设置背景   
    imagecolorallocate($this->_image, $this->bg[0], $this->bg[1], $this->bg[2]); 

    // 验证码字体随机颜色
    $this->_color = imagecolorallocate($this->_image, mt_rand(1,150), mt_rand(1,150), mt_rand(1,150));
    // 验证码使用随机字体
    $ttfPath = dirname(__FILE__) . '/Verify/' . ($this->useZh ? 'zhttfs' : 'ttfs') . '/';

    if(empty($this->fontttf)){
      $dir = dir($ttfPath);
      $ttfs = array();    
      while (false !== ($file = $dir->read())) {
        if($file[0] != '.' && substr($file, -4) == '.ttf') {
          $ttfs[] = $file;
        }
      }
      $dir->close();
      $this->fontttf = $ttfs[array_rand($ttfs)];
    } 
    $this->fontttf = $ttfPath . $this->fontttf;
    
    if($this->useImgBg) {
      $this->_background();
    }
    
    if ($this->useNoise) {
      // 绘杂点
      $this->_writeNoise();
    }
    if ($this->useCurve) {
      // 绘干扰线
      $this->_writeCurve();
    }
    
    // 绘验证码
    $code = array(); // 验证码
    $symbol=array('+','-');
    $codeNX = 0; // 验证码第N个字符的左边距
    $now_symbol=$symbol[rand(0,1)];
    for ($i = 0; $i<$this->length; $i++) {
      if($i==1){
        $code[$i] = $now_symbol;
        $codeNX += mt_rand($this->fontSize*1.2, $this->fontSize*1.6);
        imagettftext($this->_image, $this->fontSize,0, $codeNX, $this->fontSize*1.6, $this->_color, $ttfPath.'2.ttf', $code[$i]);
      }
      else{
        $code[$i] = $this->codeSet[mt_rand(0, strlen($this->codeSet)-1)];
        $codeNX += mt_rand($this->fontSize*1.2, $this->fontSize*1.6);
        imagettftext($this->_image, $this->fontSize, mt_rand(-40, 40), $codeNX, $this->fontSize*1.6, $this->_color, $this->fontttf, $code[$i]);
      } 
    }
    
    // 保存验证码
    $key    =  $this->authcode($this->seKey);
    $str=implode('', $code);
    eval("\$re=$str;");
    $code    =  $this->authcode($re);
    $secode   =  array();
    $secode['verify_code'] = $code; // 把校验码保存到session
    $secode['verify_time'] = NOW_TIME; // 验证码创建时间
    session($key.$id, $secode);
            
    header('Cache-Control: private, max-age=0, no-store, no-cache, must-revalidate');
    header('Cache-Control: post-check=0, pre-check=0', false);    
    header('Pragma: no-cache');
    header("content-type: image/png");

    // 输出图像
    imagepng($this->_image);
    imagedestroy($this->_image);
  }