$_SESSION

Creating New Session
==========================
<?php 
session_start();
/*session is started if you don't write this line can't use $_Session global variable*/
$_SESSION["newsession"]=$value;
?>
Getting Session
==========================
<?php 
session_start();
/*session is started if you don't write this line can't use $_Session global variable*/
$_SESSION["newsession"]=$value;
/*session created*/
echo $_SESSION["newsession"];
/*session was getting*/
?>
Updating Session
==========================
<?php 
session_start();
/*session is started if you don't write this line can't use $_Session global variable*/
$_SESSION["newsession"]=$value;
/*it is my new session*/
$_SESSION["newsession"]=$updatedvalue;
/*session updated*/
?>
Deleting Session
==========================
<?php 
session_start();
/*session is started if you don't write this line can't use $_Session global variable*/
$_SESSION["newsession"]=$value;
unset($_SESSION["newsession"]);
/*session deleted. if you try using this you've got an error*/
?>

PHP
<?php
// 1. Session and Database Initialization
session_start();
$host = 'localhost';
$db = 'database_name';
$user = 'username';
$pass = 'password';
$dsn = "mysql:host=$host;dbname=$db;charset=utf8mb4";
$options = [
 PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION,
 PDO::ATTR_DEFAULT_FETCH_MODE => PDO::FETCH_ASSOC,
];
try {
 $pdo = new PDO($dsn, $user, $pass, $options);
} catch (\PDOException $e) {
 throw new \PDOException($e->getMessage(), (int)$e->getCode());
}
// 2. Authentication Logic (Login)
if ($_SERVER['REQUEST_METHOD'] === 'POST' && isset($_POST['action_login'])) {
 $email = trim($_POST['email']);
 $password = $_POST['password'];
 $stmt = $pdo->prepare('SELECT id, password_hash FROM users WHERE email = ?');
 $stmt->execute([$email]);
 $user = $stmt->fetch();
 if ($user && password_verify($password, $user['password_hash'])) {
 // Regenerate session ID to prevent session fixation attacks
 session_regenerate_id(true);
 
 $_SESSION['auth_user_id'] = $user['id'];
 $_SESSION['auth_logged_in'] = true;
 
 header('Location: dashboard.php');
 exit;
 } else {
 echo "Invalid email or password.";
 }
}
// 3. Verification Condition (Check if user is logged in)
if (!isset($_SESSION['auth_logged_in']) || $_SESSION['auth_logged_in'] !== true) {
 // User is not authenticated, redirect to login page
 header('Location: login.php');
 exit;
}
// 4. Accessible only to authenticated users
echo "Access granted. User ID: " . htmlspecialchars($_SESSION['auth_user_id']);
?>

Please note that if you have register_globals to On, global variables associated to $_SESSION variables are references, so this may lead to some weird situations.
<?php
session_start();
$_SESSION['test'] = 42;
$test = 43;
echo $_SESSION['test'];
?>

Load the page, OK it displays 42, reload the page... it displays 43.
The solution is to do this after each time you do a session_start() :
<?php
if (ini_get('register_globals'))
{
 foreach ($_SESSION as $key=>$value)
 {
 if (isset($GLOBALS[$key]))
 unset($GLOBALS[$key]);
 }
}
?>