一个空对象简单的问题

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<title>系统设置</title>
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<div class="nav"><img src="images/place.jpg" width="20" height="19" />
  <div class="navname">当前位置：登录 &gt;&gt; 系统设置</div>
</div><table width="94%" border="0" align="center" cellpadding="0" cellspacing="0">
  <tr>
    <td height="3" background="images/xian.jpg"></td>
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</table><div id="container">
  <div id="title">
    <ul>
      <li id="tag1"><a href="#1" onclick="switchTag('tag1','content1');this.blur();" class="selectli1"><span class="selectspan1">基本设置</a></li>
      <li id="tag2"><a href="#2" onclick="switchTag('tag2','content2');this.blur();"><span>网络设置</a></li>
      
    </ul>
  </div>
<div id="content" class="content1">
  <div id="content1">
     <table width="87%" height="53" border="0" align="center" cellpadding="0" cellspacing="1">
                <tr>
                  <td height="51" align="center" >
                  
                  <input name="Submit3" type="submit" class="btn" value="确　定"  /></td>
                  </tr>
                
              </table> </form>    </td>
          </tr>
           
        </table>
          
        </fieldset></td>
        <td width="50%">
          <fieldset>
          <legend> 默认值</legend>
          <br />
          <table width="80%" height="125" border="0" align="center" cellpadding="0" cellspacing="0" >
            <tr>
              <td height="125" ><form method="post" action="setdisk.php">
                  <table width="98%" height="67" border="0" align="center" cellpadding="0" cellspacing="1">
                    <tr>
                      <td width="51%" height="33" align="right" >每用户默认分配空间：</td> <td width="49%" align="left" ><input name="diskspace" type="text" id="textfield"  value="<?php echo $buffer;?>"size="6" />
                      G</td>
                    </tr>
                  </table>
                <table width="87%" height="53" border="0" align="center" cellpadding="0" cellspacing="1">
                    <tr>
                      <td height="51" align="center" ><input type="submit" name="search2"  value="确　定"class="btn"/></td>
                    </tr>
                  </table>
              </form></td>
            </tr>
          </table>
          </fieldset></td></tr>
        </table>
</div>    
  
  <div id="content2" class="hidecontent">
  <table width="100%" height="207" border="0" cellpadding="0" cellspacing="0">
      <tr>
        <td width="50%"><fieldset>
          <legend> 网络设置 </legend>
         
          
          <table width="90%" height="125" border="0" align="center" cellpadding="0" cellspacing="0" >
            <tr><FORM name="admin" method="post" action="server.php" onsubmit="return check()">
              <td height="125" ><table width="92%" height="73" border="0" align="center" cellpadding="0" cellspacing="0" >
                <tr>
                  <td width="52%" height="24" align="right" >&nbsp;
                      <input name="r1" type="radio" value="0" id="dhcp1"  onclick="change(true)"  <?php if ($dhcp=='1') echo 'checked="checked"';?>>
                    启用DHCP</td>
                  <td align="center" >&nbsp;
                      <input name="r1" type="radio" value="1" id="dhcp2"  onclick="change(false)" <?php if ($dhcp=='0') echo 'checked="checked"';?> >
                    不启用DHCP</td>                </tr>
  <td colspan="2"><table width="100%" border="0" cellpadding="0" cellspacing="0" id="con">
    <tr>
      <td width="52%" height="24" align="right" >IP地址：</td>
      <td width="48%" align="left" ><input name="ip" type="text"  id="ip" value="<?php echo $ip?>"></td>
    </tr>
    <tr>
      <td width="52%" height="24" align="right" >子网掩码：</td>
      <td width="48%" align="left" ><input name="netmask" type="text"  id="netmask" value="<?php echo $netmask?>"></td>
    </tr>
    <tr>
      <td width="52%" height="24" align="right" >默认网关：</td>
      <td width="48%" align="left" ><input name="gateway" type="text"  id="gateway" value="<?php echo $gateway?>" ></td>
    </tr>
  </table></td><tr>
    <td width="52%" height="24" align="right" >服务器名：</td>
    <td align="left" ><input name="servername" type="text" id="servername"    value="<?php $servername = iconv("utf-8",'gb2312',$servername); echo $servername;?>"  onKeydown="return check3(event)"/></td>
  </tr>
              </table>
              <table width="87%" height="24" border="0" align="center" cellpadding="0" cellspacing="1">
                    <tr>
                      <td height="25" align="center" ><input name="tijiao" type="submit" class="btn"  value="确　定" >&nbsp;                        </td>
                    </tr>
                </table></td></FORM>
            </tr>
          </table>
          
          <br />
        </fieldset></td>
</tr>
</table>
</div></div>
</div>
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