js 无法给 a href赋值，帮我找找问题吧

在另一个页面里面试了可以，但是这里就不行了不知道为什么了，老是提示 为空或者不是对象
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<link type="text/css" href="css/main.css"  rel="stylesheet"/>
<script type="text/javascript" src="js/yccd.js"></script>
<!--script type="text/javascript" src="js/eimglink_js.php"></script-->
<title>无标题文档</title>
</head>
<?php
include("class/show_info_class.php");
include("config/config.php");
$w = new index_info($host,$username,$userpass,$dbname);
?>
<body>
<center>
<div class="main">
<div class="header">
      <div class="logo"></div>
      <!------导航开始------>
      <div class="daohang">
        <div class="cpzs" onmouseover="cpzs()"><a href="#"></a></div>
        <div><a href="#" class="khzq"></a></div>
        <div><a href="fwzx.php" class="fwzx"></a></div>
        <div><a href="edit/gynb.php" class="gynb"></a></div>
        <div><a href="liuyan.php" class="lyb"></a></div>
      </div>
      <!--------导航结束---------->
    </div>
    <!-------------隐藏选项卡开始---------------->
    <div id="bigyc">
  <?php
          $w->pinpai();
          ?>      
          </div> 
    <!-----------隐藏算显卡结束--------->
   <!-------------八图展示开始----------------->
   <div id="eight">
      <script type="text/javascript">
  eightimg();
  eight();
      </script>
     <a href="#" id="biglink">&nbsp;sddfsdfsdfsfs</a>   我要动态改变这个链接的href属性
     <table border="0" cellpadding="0" cellspacing="0" class="elink">
        <tr>
           <td><a href="#" onmouseover="divimg('1')" id="link1"></a></td>
           <td><a href="#" onmouseover="divimg('2')" id="link2"></a></td>
           <td><a href="#" onmouseover="divimg('3')" id="link3"></a></td>
           <td><a href="#" onmouseover="divimg('4')" id="link4"></a></td>
           <td><a href="#" onmouseover="divimg('5')" id="link5"></a></td>
           <td><a href="#" onmouseover="divimg('6')" id="link6"></a></td>
           <td><a href="#" onmouseover="divimg('7')" id="link7"></a></td>
           <td><a href="#" onmouseover="divimg('8')" id="link8"></a></td>
        </tr>
     </table>
   </div> 
   <!---------------八图展示结束-----------------------> 
   <!--------------中间部分开始--------------------->
     <div class="middle">
        <div class="mleft">
           <div class="cpbj">
             <div class="cpbjtop"></div>
             <div class="cpbjnr">
                <?php
                $w->cpbj();
?>
             </div>
            </div> 
            <div class="news" id="news">
               <a href="#"><div id="hyxw" onmouseover="news('1')"></div></a>
               <a href="#"><div id="nbgg" onmouseover="news('2')"></div></a>
               <div id="xwnr">
                <?php
                $w->news('news');
?>
               </div>
               <div id="ggnr">
                  <?php
                $w->news('gonggao');
?>
               </div>
            </div>         
          </div>
        <div class="mcenter">
           <?php
              $w->middle();
   ?>
        </div>
        <div class="mright">
           <div class="sousuo">
              <table class="sousuott">
                 <tr><td colspan="2" class="sousuoimg"></td></tr>
                 <tr><td colspan="2"></td></tr>
                 <tr><td class="sousuottl">选择产品</td><td class="sousuottr">
                 <select>
                   <option value="1"></option>
                 </select>
                 </td></tr>
                 <tr><td class="sousuottl">选择品牌</td><td class="sousuottr">
                 <select>
                   <option value="1"></option>
                 </select>
                 </td></tr>
                 <tr><td class="sousuottl">选择型号</td><td class="sousuottr">
                 <select>
                   <option value="1"></option>
                 </select>
                 </td></tr>
              </table>
           </div>
           <div class="diy">
              <?php
               $w->diy();
  ?>
           </div>
        </div>
     </div>
  <?php
include("footer.php");
?> 
</div></center>
</body>
</html>
下面的是js代码
var imgar = [];
var linkarray = [];
linkarray[0] = "http://www.baidu.com";
linkarray[1] = "http://www.google.cn";
linkarray[2] = "http://www.qq.com";
linkarray[3] = "http://www.cgxy.org.cn";
linkarray[4] = "http://www.baidu.com";
linkarray[5] = "http://www.baidu.com";
linkarray[6] = "http://www.baidu.com";
linkarray[7] = "http://www.baidu.com";
alert(linkarray);
    var i = 1;
function eightimg(){
for(i=1;i<9;i++){
  imgar[i] = new Image();
  imgar[i].src = "images/eightimg/eight_"+i+".jpg"; 
  imgar[i].onload=function(){
this.width = 960;
this.height =380;
   }
 }
}
function eight(){
divid = document.getElementById('eight');
tt = document.getElementById('biglink');
alert(linkarray[0]);
i = 1;
function bimg(){
if(i<=8){
ll = i-1;
divid.style.backgroundImage = 'url("'+imgar[i].src+'")';
alert('"'+linkarray[ll]+'"');
tt.href = '"'+linkarray[ll]+'"';
i++;
}else{
i = 1;
}  
 }
 imgb = setInterval(bimg,5000);
}
//点击变换图片
function divimg(i){
window.i = i;
img = document.getElementById('eight');
img.style.backgroundImage = 'url("'+imgar[i].src+'")';
}