验证码在哪加时间的字符串才可以避免读缓存解决方法

验证码在哪加时间的字符串才可以避免读缓存
这是我用类包装的一个验证图片功能 但是我不知道在哪加时间字符串才可以避免读缓存 还请高手指教下

PHP code

<!--

Code highlighting produced by Actipro CodeHighlighter (freeware)
http://www.CodeHighlighter.com/

-->
<?php class ImageCode{
    private $randcode;                                            //随机字符
    private $randcodelength;                                    //随机字符长度
    private $imgWidth;                                            //随机字符宽度
    private $imgHeight;                                        //随机字符高度
    private $line = 5;                                                //干扰线
    private $code = array("q","w","e","r","t","y","u","i","o","p","a","s","d","f","g","h","j","k","l","z","x","c","v","b","n","m",
                            "Q","W","E","R","T","Y","U","I","O","P","A","S","D","F","G","H","J","K","L","Z","X","C","V","B","N","M",
                            "1","2","3","4","5","6","7","8","9","0");
    private $codelength;                                        //数组的长度

    private $image;                                            //图片
    private $background;                                        //背景
    private $foreground;                                        //前景

    function __construct(){
        $this->randcodelength = 5;
        $this->imgWidth = 80;
        $this->imgHeight = 20;
        $this->codelength = count($this->code)-1;                //获取数组长度当随机范围

        $this->createrandcode();
        $this->createimg();
        $this->createString();
        $this->drawline($this->line);
        $this->ending();

    }
    function createrandcode(){
        for($i=0;$irandcodelength;$i++){                //初始化验证码
            $this->randcode .= $this->code[rand(0,$this->codelength)];
        }
    }
    function createimg(){                                        //创建图片 前景 背景颜色
        $this->image = imagecreatetruecolor($this->imgWidth,$this->imgHeight);
        $this->background = imagecolorallocate($this->image,0,0,0);
        $this->foreground = imagecolorallocate($this->image,255,255,255);
    }
    function createString(){                                    //创建字符串
        imagestring($this->image,5,rand(5,35),0,$this->randcode,$this->foreground);
    }
    function drawline($some){
        for($i=0;$iimage,rand(1,$this->imgWidth),rand(1,$this->imgHeight),rand(1,$this->imgWidth),rand(1,$this->imgHeight),$this->foreground);
        }
    }
    function ending(){
        ob_clean();                                    //抛弃缓存内容
        header("Content-type: image/jpeg");
        imagejpeg($this->image);
    }
}
?>

------解决方案--------------------
网上搜一大把。
<script><br />function RefreshCode(obj){<br /> obj.src = obj.src + "?code=" + Math.random();<br />}<br /></script>