How to use PHP verification code class

<?php
/**
* @version 1.0
* @author  bolted snail
* @date 2011-10-15
* @PHP验证码类
* 使用方法：
* $image=new Captcha();
* $image->config(&#39;宽度&#39;,&#39;高度&#39;,&#39;字符个数&#39;,&#39;验证码session索引&#39;);
* $image->create();//这样就会向浏览器输出一张图片
* //所有参数都可以省略，
* 默认是：宽80 高20 字符数4 验证码session索引captcha_code
* 第四个参数即把验证码存到$_SESSION[&#39;captcha_code&#39;]
* 最简单使用示例:
* $image=new Captcha();
* $image->create();//这样就会向浏览器输出一张图片
*/
class Captcha
{
private $width=80,$height=20,$codenum=4;
public $checkcode;   //产生的验证码
private $checkimage;  //验证码图片 
private $disturbColor = &#39;&#39;; //干扰像素
private $session_flag=&#39;captcha_code&#39;;//存到session中的索引
//尝试开始session
function __construct(){
  @session_start();
}
/*
* 参数：（宽度，高度，字符个数）
*/
function config($width=&#39;80&#39;,$height=&#39;20&#39;,$codenum=&#39;4&#39;,$session_flag=&#39;captcha_code&#39;)
{ 
  $this->width=$width;
  $this->height=$height;
  $this->codenum=$codenum;
  $this->session_flag=$session_flag;
}
function create()
{
  //输出头
  $this->outFileHeader();
  //产生验证码
  $this->createCode();
  //产生图片
  $this->createImage();
  //设置干扰像素
  $this->setDisturbColor();
  //往图片上写验证码
  $this->writeCheckCodeToImage();
  imagepng($this->checkimage);
  imagedestroy($this->checkimage);
  $_SESSION[$this->session_flag]=$this->checkcode;
}
/*
  * @brief 输出头
  */
private function outFileHeader()
{
  header ("Content-type: image/png");
}
/**
  * 产生验证码
  */
private function createCode()
{
  $this->checkcode = strtoupper(substr(md5(rand()),0,$this->codenum));
}
/**
  * 产生验证码图片
  */
private function createImage()
{
  $this->checkimage = @imagecreate($this->width,$this->height);
  $back = imagecolorallocate($this->checkimage,255,255,255); 
  $border = imagecolorallocate($this->checkimage,0,0,0); 
  imagefilledrectangle($this->checkimage,0,0,$this->width - 1,$this->height - 1,$back); // 白色底
  imagerectangle($this->checkimage,0,0,$this->width - 1,$this->height - 1,$border);  // 黑色边框
}
/**
  * 设置图片的干扰像素 
  */
private function setDisturbColor()
{
  for ($i=0;$i<=200;$i++)
  {
  $this->disturbColor = imagecolorallocate($this->checkimage, rand(0,255), rand(0,255), rand(0,255));
  imagesetpixel($this->checkimage,rand(2,128),rand(2,38),$this->disturbColor);
  }
}
/**
  *
  * 在验证码图片上逐个画上验证码
  *
  */
private function writeCheckCodeToImage()
{
  for ($i=0;$i<$this->codenum;$i++)
  {
  $bg_color = imagecolorallocate ($this->checkimage, rand(0,255), rand(0,128), rand(0,255));
  $x = floor($this->width/$this->codenum)*$i;
  $y = rand(0,$this->height-15);
  imagechar ($this->checkimage, rand(5,8), $x+5, $y, $this->checkcode[$i], $bg_color);
  }
}
function __destruct()
{
  unset($this->width,$this->height,$this->codenum,$this->session_flag);
}
}
?>