Session in php

<?php
session_start();
include_once("classes/conn.php");

//$page_id="ADMIN_INDEX";

if(isset($_POST['username']) && isset($_POST['password']))
{
$username=$_POST['username'];
$password=$_POST['password'];
//echo $username.' '.$password;
$result=mysql_query("select * from admin_users where username='$username' and password='$password'");
$page_id="ADMIN_INDEX";
if($row=mysql_fetch_array($result))
    {
        $login=true;
        $_SESSION['admin_id']=$row['username'];
        $_SESSION['USERNAME']=$row['username'];

    }
if($login)
{

header("Location: dashboard.php");
}
else
{
    header("Location: index.php?message=Incorrect login id/password");
   
}
}   
?>
<form action="index.php" method="post">
<label>UserName :</label>
<input type="text" name="username"/><br />
<label>Password :</label>
<input type="password" name="password"/><br />
<input type="submit" value=" Submit "/><br />
</form>







call in all page

<?php
session_start();
if(!$_SESSION['admin_id'] && $page_id!="ADMIN_INDEX")
{
    exit;
}


?>