Core PHP: $_SESSION (7.7)

Sessions

Using a session, you can store information in variables, to be used across multiple pages.
Information is not stored on the user's computer, as it is with cookies.
By default, session variables last until the user closes the browser.

Start a PHP Session

A session is started using the session_start() function.
Use the PHP global $_SESSION to set session variables.

<?php
// Start the session
session_start();

$_SESSION['color'] = "red";
$_SESSION['name'] = "John";
?>

Now, the color and name session variables are accessible on multiple pages, throughout the entire session.

The session_start() function must be the very first thing in your document. Before any HTML tags.