Working with Sessions & Cookies

Since each request is normally independent (from How PHP Code Runs), sessions let data persist across requests for the same visitor, identified by a cookie PHP sets automatically.

session_start(); // must run before any output

$_SESSION['username'] = 'Ada';
echo $_SESSION['username']; // available on later requests too

setcookie('theme', 'dark', time() + 3600); // expires in 1 hour
echo $_COOKIE['theme'] ?? 'light';

Common mistakes

• Calling session_start() after any HTML or echo has already been sent — this throws a "headers already sent" error, since session cookies must be set before any response body.