PHP - Session Help

To my understanding, a session is created and then stored on the server with its assigned value and a unique id. So, if that is correct I should be able to somehow locate a particular session on the server based on it's assigned value. Well, I was hoping to use this concept to keep multiple people from logging in under the same account at the same time.

I figured that I could check this in my login script by declaring the customer's id as the session value when they login. Then, I could check for a session variable equal to the cusotmer's id when they try to login. My (untested) code is below. Am I going about this right, and how would I check to see if a user's session is currently set on the server?

Code: [Select]

<?php
// initiate session and redirect logged in users
session_start();
if(isset($_SESSION['customer_id'])) {
header('location:my_videos.php');
}

// if login button was pressed
if(array_key_exists('login', $_POST)) {
// initalize error array and check that user supplied a username and password
$error = array();
$username = trim($_POST['username']);
$password = trim($_POST['password']);
if(empty($username)) {
$error['username'] = 'Please enter your username.';
}
if(empty($password)) {
$error['password'] = 'Please enter your password.';
}
// if username and password supplied then proceed
if(!$error) {
// connect to the database
require_once('includes/connect.php');
// filter data for query
$username = mysql_real_escape_string($username);
$password = md5(mysql_real_escape_string($password));
$queryUser = mysql_query("SELECT customer_id, customer_username, customer_password FROM customer WHERE customer_username = '$username' AND customer_password = '$password'", $connect) or die(mysql_error());
$dataUser = mysql_fetch_assoc($queryUser);
$rowsUser = mysql_num_rows($queryUser);
$customerId = $dataUser['customer_id'];
// determine if the user is a valid customer
if($rowsUser == 1) {
// see how many IP addresses the customer has used to login with in the past 24 hours
$queryIP = mysql_query("SELECT COUNT(DISTINCT log_ip) AS ip FROM log WHERE log_customer_id = $customerId AND log_timestamp IN((DATE_SUB(NOW(), INTERVAL 1 DAY)), NOW())") or die(mysql_error());
$dataIP = mysql_fetch_assoc($queryIP);
if($dataIP['ip'] > 3) {
$error['ip'] = 'This customer account has reached the maximum number of IP addresses allowed. If you feel this is a system error please send us an email via the Contact Us form.';
exit;
} else {
// see if the customer is already logged in
$queryLogged = mysql_query("SELECT customer_id, customer_logged_in FROM customer WHERE customer_logged_in = 1 AND customer_id = '$customer_id'", $connect) or die(mysql_error());
$dataLogged = mysql_fetch_assoc($queryLogged);
$rowsLogged = mysql_num_rows($queryLogged);
if($rowsLogged == 1) { // if database shows the customer is already logged in
// if there is also a session variable set that matches their customer id on the server
if($_SESSION['customer_id'] == $customerId) { // i need to somehow find this session value on the server first
// this means the user is trying to login from two different locations
header('location:bad_login.php');
exit;
// if no session variable for customer id is set on the server
} else {
// this means user lost connection without logging out
// set a customer id session variable
$_SESSION['customer_id'] = $customerId;
// log customer activity
$ip = $_SERVER["REMOTE_ADDR"];
$queryLog = mysql_query("INSERT INTO log (log_timestamp, log_ip, log_customer_id) VALUES (NOW(), '$ip', '$customerId')", $connect)or die(mysql_error());
// send user to appropriate page (if a previous page session variable exists send them there)
if(isset($_SESSION['previous_page'])) {
header('location:video_info.php');
// if not send them to the my_videos.php page
} else {
header('location:my_videos.php');
}
}
} 
// if database shows the customer is not logged in
else {
$_SESSION['customer_id'] = $customerId;
$queryLogin = mysql_query("UPDATE customer SET customer_logged_in = 1 WHERE customer_id = '$customerId'", $connect) or die(mysql_error());
$ip = $_SERVER["REMOTE_ADDR"];
$queryLog = mysql_query("INSERT INTO log (log_timestamp, log_ip, log_member_id) VALUES (NOW(), '$ip', '$customerId')", $connect)or die(mysql_error());
if(isset($_SESSION['previous_page'])) {
header('location:video_info.php');
} else {
header('location:my_videos.php');
}
}
}
// if there was no match found in the database
} else {
$error['login'] = "Incorrect username and/or password. If you do not have an account with us, please create one";
}
}
}
?>