PHP - Guardar resultado de consulta en mysql en una variable de sesion

SELECT id_usuario FROM `usuarios` WHERE email = 'david'

<?php

    /* start the session */

    session_start();

     $host_db = "localhost";

     $user_db = "root";

     $pass_db = "dmrx921";

     $db_name = "despliegate";

     $tbl_name = "usuarios";

    // Connect to server and select databse.

    mysql_connect("$host_db", "$user_db", "$pass_db")or die("Cannot Connect to Data Base.");

    mysql_select_db("$db_name")or die("Cannot Select Data Base");

    // sent from form

    $username = $_POST['email'];

    $password = $_POST['contrasena'];

    $sql= "SELECT * FROM $tbl_name WHERE email = '$username' and contrasena='$password'";

    $result=mysql_query($sql);

    // counting table row

    $count = mysql_num_rows($result);

    // If result matched $username and $password

    if($count == 1){

     $_SESSION['loggedin'] = true;

     $_SESSION['email'] = $username;

     $_SESSION['start'] = time();

     $_SESSION['expire'] = $_SESSION['start'] + (60 * 60) ;

    header('Location: index2.php');

    }

     else {

     //echo "<br/>Email o contraseña estan incorrectos.<br>";

     echo "<a href='index.php'>Volver a Intentarlo</a>";

    }

    ?>

$_SESSION['loggedin'] = true;

$_SESSION['email'] = $username;

$_SESSION['start'] = time();

$_SESSION['expire'] = $_SESSION['start'] + (60 * 60) ;

$row=mysql_fetch_row($result);

$_SESSION['id']=$row["id"];