求大神给指点下这些代码有什么有关问题啊为什么session变量不生效

求大神给指点下这些代码有什么问题啊，为什么session变量不生效啊

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

mysql_select_db($database_yiwuyiwu, $yiwuyiwu);
$query_Recordset1 = "SELECT * FROM `user`";
$Recordset1 = mysql_query($query_Recordset1, $yiwuyiwu) or die(mysql_error());
$row_Recordset1 = mysql_fetch_assoc($Recordset1);
$totalRows_Recordset1 = mysql_num_rows($Recordset1);
?>
// *** Validate request to login to this site.
if (!isset($_SESSION)) {
  session_start();
}
// ***session变量开始了
$loginFormAction = $_SERVER['PHP_SELF'];
if (isset($_GET['accesscheck'])) {
  $_SESSION['PrevUrl'] = $_GET['accesscheck'];
}

if (isset($_POST['Username'])) {
  $loginUsername=$_POST['Username'];
  $password=(md5($_POST['Password']));
  $MM_fldUserAuthorization = "enable";
  $MM_redirectLoginSuccess = "index.php";
  $MM_redirectLoginFailed = "zhuce.php";
  $MM_redirecttoReferrer = false;
  //*** 输入账号和密码，判断转向页面。
  mysql_select_db($database_yiwuyiwu, $yiwuyiwu);
  	
  $LoginRS__query=sprintf("SELECT u_id u_name, pass, enable FROM `user` WHERE u_name=%s AND pass=%s",
  //*** 启用符号是这个“·”
  GetSQLValueString($loginUsername, "text"), GetSQLValueString($password, "text")); 
   
  $LoginRS = mysql_query($LoginRS__query, $yiwuyiwu) or die(mysql_error());
  $row_LoginRS = mysql_fetch_assoc($LoginRS);
  $loginFoundUser = mysql_num_rows($LoginRS);
  
  
  if ($loginFoundUser) {
    
    $loginStrGroup  = mysql_result($LoginRS,0,'enable');
    
    //***   声明三个会话变量并分配值
    $_SESSION['MM_Username'] = $loginUsername;
    $_SESSION['MM_UserGroup'] = $loginStrGroup;	
	$_SESSION['u_id']= $row_LoginRS('u_id');     

    if (isset($_SESSION['PrevUrl']) && false) {
      $MM_redirectLoginSuccess = $_SESSION['PrevUrl'];	
    }
    header("Location: " . $MM_redirectLoginSuccess );
  }
  else {
    header("Location: ". $MM_redirectLoginFailed );
  }
}
?>
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