前辈，帮解决下这个有关问题

前辈，帮解决下这个问题
我做了一个登录用户的页面，做好之后，在纯页面的情况下，我给它了一个登录用户验证。然后在上面生成了这些代码

if (!function_exists("GetSQLValueString")) {
function GetSQLValueString($theValue, $theType, $theDefinedValue = "", $theNotDefinedValue = "") 
{
  if (PHP_VERSION   $theValue = get_magic_quotes_gpc() ? stripslashes($theValue) : $theValue;
  }

  $theValue = function_exists("mysql_real_escape_string") ? mysql_real_escape_string($theValue) : mysql_escape_string($theValue);

  switch ($theType) {
  case "text":
  $theValue = ($theValue != "") ? "'" . $theValue . "'" : "NULL";
  break;  
  case "long":
  case "int":
  $theValue = ($theValue != "") ? intval($theValue) : "NULL";
  break;
  case "double":
  $theValue = ($theValue != "") ? doubleval($theValue) : "NULL";
  break;
  case "date":
  $theValue = ($theValue != "") ? "'" . $theValue . "'" : "NULL";
  break;
  case "defined":
  $theValue = ($theValue != "") ? $theDefinedValue : $theNotDefinedValue;
  break;
  }
  return $theValue;
}
}
?>
// *** Validate request to login to this site.
if (!isset($_SESSION)) {
  session_start();
}

$loginFormAction = $_SERVER['PHP_SELF'];
if (isset($_GET['accesscheck'])) {
  $_SESSION['PrevUrl'] = $_GET['accesscheck'];
}

if (isset($_POST['username'])) {
  $loginUsername=$_POST['username'];
  $password=$_POST['userpsw'];
  $MM_fldUserAuthorization = "";
  $MM_redirectLoginSuccess = "admin.php";
  $MM_redirectLoginFailed = "index.php";
  $MM_redirecttoReferrer = true;
  mysql_select_db($database_boardConn, $boardConn);
  
  $LoginRS__query=sprintf("SELECT username, userpsw FROM board_admin WHERE username=%s AND userpsw=%s",
  GetSQLValueString($loginUsername, "text"), GetSQLValueString($password, "text")); 
   
  $LoginRS = mysql_query($LoginRS__query, $boardConn) or die(mysql_error());
  $loginFoundUser = mysql_num_rows($LoginRS);
  if ($loginFoundUser) {
  $loginStrGroup = "";
   
if (PHP_VERSION >= 5.1) {session_regenerate_id(true);} else {session_regenerate_id();}
  //declare two session variables and assign them
  $_SESSION['MM_Username'] = $loginUsername;
  $_SESSION['MM_UserGroup'] = $loginStrGroup;  

  if (isset($_SESSION['PrevUrl']) && true) {
  $MM_redirectLoginSuccess = $_SESSION['PrevUrl'];
  }
  header("Location: " . $MM_redirectLoginSuccess );
  }
  else {
  header("Location: ". $MM_redirectLoginFailed );
  }
}
?>
但是我一浏览
页面就出现这样的错误：
Warning: session_start() [function.session-start]: Cannot send session cache limiter - headers already sent (output started at E:\AppServ\www\webBoard2\login.php:2) in E:\AppServ\www\webBoard2\login.php on line 5

以前也预见这样问题，那会把// *** Validate request to login to this site.
if (!isset($_SESSION)) {
  session_start();
}
这一段的PHP代码放到上面就行了，这次不知道怎么还是不行，帮忙 解决下，谢谢



------解决方案--------------------