Help in session_start

Hi,

Why the session start can’t call my session variable…it does not echo my session variable.


<?php
session_start();
if(isset($_SESSION['username'])){
include_once("conn.php");
?>
<!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" xml:lang="en-us">
<head>
</head>
<body>
<p><?php echo  $_SESSION['username'];?></p>
<?php
$tag=mssql_query("Select * from tblUser where UserName='$_SESSION[username]'");
if($_POST['Save']){
$oldpass=md5($_POST['oldpass']); 

	$rpass=mssql_fetch_assoc(mssql_query("Select * from tblUser"));
	if($oldpass==$rpass['Pass']){
		//mssql_query("sp_userpass @newpass='$_POST[newpass]',@tag='0'");
		echo "Save";
	}else{
		echo"<font color='#FF0000'>Old Password not match</font>";
		
	}

}
?>
<form method='post' action=''>
<table >
	<tr>
		<td align='right'>Old Password :</td><td align='left'> <input type='password' name='oldpass' ></td>
	</tr>
	<tr>
		<td align='right'>New Password : </td><td align='left'><input type='password' name='npass' ></td>
	</tr>
	<tr>
		<td align='right'>Re-Enter Password :</td><td align='left'> <input type='password' name='newpass' ></td>
	</tr>
	<tr>
		<td align='right'></td>
		<td>
			<input type='submit' name='Save' value='Save'>
			<input type='Reset' name='Reset' value='Reset'>
		</td>	
	</tr>
</table>
</form>
</body>
</html>
<?php
}else{
	header('location:login.php');
}
?>

Where does the session variable get set?

thanks for the reply…
i just figure it out on my login form…