Sessions und Cookies in PHP

<?php

// Start session (must be before any output)
session_start();

// Store data
$_SESSION["user_id"] = 42;
$_SESSION["username"] = "alice";

// Read data
if (isset($_SESSION["user_id"])) {
    echo "Welcome, " . $_SESSION["username"];
}

// Flash message pattern
$_SESSION["flash"] = "Saved successfully!";
// ... redirect, then on next request:
if (isset($_SESSION["flash"])) {
    echo $_SESSION["flash"];
    unset($_SESSION["flash"]);
}

// Destroy session
session_unset();   // clear variables
session_destroy(); // destroy session

// Set cookie
setcookie("theme", "dark", time() + 86400, "/");