简单的session问题

[Cain]

下面这个文件被N个文件调用,用户登陆登出都是正常的,可是为什么转到其他页面之后session的值就没了呢?
<?
//用户登陆
if($action == 'logout')
{
$_SESSION['USER'] = '';
$_SESSION['PWD'] = '';
$_SESSION['TYPE'] = '';
header('Location:index.php');
}
if(!empty($_SESSION['USER']) && !empty($_SESSION['PWD'])){
$echo.="<table cellpadding=0 cellspacing=0 border=0><tr><td class='unnamed2'>  $user 欢迎您!</td></tr>
<tr><td class='unnamed2'现在是北京时间:</td></tr>
</tr><td class='unnamed2'>".date('Y-m-d H:i:s')."</td></tr>
<tr><td class='unnamed2'><a href='login.php?action=logout'>登出</a></td></tr></table>";
}
elseif(!empty($_POST['login']))
{
$Str="select username,password,type from new_user where username='".$user_name."' and password='".$password."'";
$Rst=mysql_query($Str,$Server) or die($Str);
$Num=mysql_num_rows($Rst);
$Row=mysql_fetch_object($Rst);
$user=$user_name;
$type=$Row->type;
if($Num == 0)
{
header('Location:index.php?action=error');
}
else
{
$_SESSION['USER'] = $username;
$_SESSION['PWD'] = $password;
$_SESSION['TYPE'] = $type;
$echo.="<table cellpadding=0 cellspacing=0 border=0><tr><td class='unnamed2'>  $user 欢迎您!</td></tr>
<tr><td class='unnamed2'>现在是北京时间:</td></tr>
<tr><td class='unnamed2'>".date('Y-m-d H:i:s')."</td></tr>
<tr><td class='unnamed2'><a href='login.php?action=logout'>登出</a></td></tr></table>
";
}
}
else
{
$echo = $login;
}
?>

[大白菜芯]

对 的 .每个都 要 用 session_start() 打开

[afoskoo]

其他页面要使用session要用session_start()打开session.