Login - Criar $_SESSION

<?php
include("../require/db_connection.php");
include("top_foot/header.php");
if(isset($_GET['msg'])){
if($_GET['msg']=="error"){
echo ' <span class="notification n-error" style="margin:auto; margin-top:2%; width:50%;">Error!! Wrong username or password!!</span>';
}
elseif($_GET['msg']=="logout"){
echo ' <span class="notification n-success" style="margin:auto; margin-top:2%; width:50%;">Logged out successfully!!</span>';
}
}
?>
<form method="post" action="login.php" style="margin-top:7%; margin-left:25%;">
<table id="login_form" width="100%" border="0" align="center" cellpadding="4" cellspacing="4">
<tr>
<td>
<!--<img src="../images/" alt="" title="" />-->
</td>
</tr>

<tr>
<td width="20%" align="right" valign="middle">Username</td>
<td width="3%" align="left" valign="middle">:</td>
<td width="77%" align="left" valign="middle">
<label>
         <input name="login" type="text" style="width:200px; height:15px; padding:5px;" />
</label>
</td>
</tr>

<tr>
<td align="right" valign="middle">Password</td>
<td align="left" valign="middle">:</td>
<td align="left" valign="middle">
<label>
         <input name="password" type="password" style="width:202px; height:15px; padding:5px;" />
</label>
</td>
</tr>

<tr>
<td align="left" valign="middle"> </td>
<td align="left" valign="middle"> </td>
<td align="left" valign="middle">
<label>
         <input type="submit" name="button" id="button" value="Login" />
</label>
</td>
</tr>
</table>
</form>
<?php
include("top_foot/footer.php");
?>

<?php
ob_start();
require('../require/db_connection.php');
// Define $myusername and $mypassword
$login = $_POST['login'];
$password = $_POST['password'];
// To protect MySQL injection (more detail about MySQL injection)
$myusername = mysql_real_escape_string($login);
$mypassword = sha1($password);

$sql="SELECT * FROM admin WHERE admin_user='".$myusername."' and admin_password='".$mypassword."'";
$result=mysql_query($sql) or die(mysql_error());
$rows=mysql_fetch_assoc($result);

// Mysql_num_row is counting table row
$count=mysql_num_rows($result);
if($count==1){
$_SESSION['user']=$rows['admin_user'];
header("location:pages/dashboard.php");
}
else {
header("location:index.php?msg=error");
}
ob_end_flush();
?>

<?php
session_start();
if(!$_SESSION['user']){
echo $_SESSION['user']; //da erro nesta linha, nao conhece 'user'
die();
}

include('top_foot/header.php');
?>

<!-- Header. Main part -->
<div id="header-main">
<div class="container_12">
<div class="grid_12">
<div id="logo">
         <ul id="nav">
         <li id="current"><a href="">Dashboard</a></li>
         <li><a href="">Users</a></li>
         <li><a href="">Banners</a></li>
         <li><a href="">Bands</a></li>
         <li><a href="">News</a></li>
<!-- CONTINUA (....) -->