php 页面能正常访问，但是一刷新就成空白页面了

在地址栏直接输入地址可以访问，没有问题。但是只要一刷新页面就变成空白页了。
查看原代码，源代码都在。在html 代码里有一段javascript 程序 
<script language="javascript">
if(self!=top){top.location=self.location;}

function sf(){document.login.username.focus();}
 </script>。
只要把这段去掉就一切正常。反之则刷新成空白页面了。如果把这段代码放到程序的最后也能一切正常。页面编码设置也设了
<META http-equiv=Content-Type content="text/html; charset=gb2312">
还是不行。<?php
include '../include/mysql_class.php';
$sql = "select websitename from compinfo";
$result = $db->query($sql);
$row = $db->fetch_array($result);
$WebSiteName = $row["websitename"];
$db->close();

?>
<!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=gb2312">
<link rel="stylesheet" href="images/css.css" type="text/css"><script language="javascript">
if(self!=top){top.location=self.location;}

function sf(){document.login.username.focus();}
 </script>
<title>SiteEngine 登录</title>
</head><body onLoad=sf()>
<table width="100%" align=center height="100%" border="0">
    <tr>
      <td valign=top bgcolor="#F8F8F8"><br>      

<form method="post" action="login.php" name="login">
  <table width="350" border="0" align="center"  cellpadding="0" cellspacing="0">
  <tr>
  <td background="images/Login_1.gif">&nbsp;</td>
  </tr>
  <tr>
  <td height="77" align="center" background="images/Login_2.gif"><font color=#ffffff><?php echo $WebSiteName ?></font></td>
  </tr>   
  <tr>
  <td height="148" align="center" background="images/Login_3.gif">
  <table width="100%"  border="0" cellpadding="0" cellspacing="0">
                <tr>
                  <td width="30%">&nbsp;</td>
                  <td width="70%">&nbsp;</td>
                </tr>
                <tr>
                  <td align="right">管理用户</td>
                  <td align="left"><input type="text" name="username" size="16"  onMouseOver ="this.style.backgroundColor='#879BD8'" onMouseOut ="this.style.backgroundColor='#f8f8f8'"></td>
                </tr>
                <tr>
                  <td align="right">管理密码</td>
                  <td align="left"><input type="password"  name="password" size="16" onMouseOver ="this.style.backgroundColor='#879BD8'" onMouseOut ="this.style.backgroundColor='#f8f8f8'"></td>
                </tr>
                <tr>
                  <td height="20" align="right">输入验证</td>
                  <td height="20" align="left">     <input name="check_image" type="text" id="check_image" size="4" maxlength="4" />     <img src="../code.php" alt="看不清，请点刷新！" width="50" height="20" align="middle" id="code" style="cursor:hand;" onClick="document.getElementById('code').src='../code.php?'+Math.random();">
                  <a onClick="document.getElementById('code').src='../code.php?'+Math.random();" style="cursor:hand;" >刷新</a> </td>
                </tr>
                <tr align="center">
                  <td colspan="2"><input type="submit" name="submit" class="Textborder" value="确定" onmouseover ="this.style.backgroundColor='#879BD8'" onmouseout ="this.style.backgroundColor='#cccccc'" />
                    <input type="reset" name="reset" class="Textborder" value="取消" onmouseover ="this.style.backgroundColor='#879BD8'" onmouseout ="this.style.backgroundColor='#cccccc'" /></td>
                </tr>
              </table>
    </td>
  </tr>   
  </table>
</form>
  </td>
    </tr>
</table>
</div>
</body>