How to check is someone is online? - php

Question

So i have made a simple log in system in php how would i go on making a function on php to check if someone is online also can it be done in php
login system
<?php
if (empty($_POST) === false) {
$username = $_POST['username'];
$password = $_POST['password'];
if (empty($username) === true || empty($password) === true) {
$errors[] = 'Please Enter a username and password';
} else if (user_exists($username) === false) {
$errors[] ='Sorry but the user you entered dose not exist';
} else if (user_active($username) === false) {
$errors[] ='You have not activated your account please check your email to activate your account';
} else {
if (strlen($password) > 32){
$errors[] ='Passwords to long !';
}
$login = login($username, $password);
if ($login === false) {
$errors[] ='incorrect username or password';
} else {
$_SESSION['user_id'] = $login;
header('Location: index.php');
exit();
}
}
} else {
$errors[] = 'No information received';
}
echo output_errors($errors);
?>

Answer 1

You can use sessions for this:
http://php.net/manual/en/book.session.php
Simple usage of sessions:
// Start the session
if(!session_id())
session_start();
$_SESSION["the_user"] = true;
// To check if the session is alive
if(!empty($_SESSION["the_user"]))
echo "User is online !";
else
echo "User isn't online!";
// Delete the session
unset($_SESSION["the_user"]);
Note that this is just a simple usage of the session, the session will be alive even if the user went of the site. but it will be for a few minutes. (session's expire time)