How to keep data in the session

Short answer

Call session_start() and work with the $_SESSION array:

<?php
 
session_start();
 
$_SESSION['cart'][] = 42;
 
echo count($_SESSION['cart']);

Removing a value

unset() removes a single value:

<?php
 
session_start();
 
unset($_SESSION['cart']);

Ending the session

When the user logs out, clear the array and destroy the session:

<?php
 
session_start();
 
$_SESSION = [];
session_destroy();

Functions used in this recipe

session_start — Starts a session or resumes the existing one.
session_destroy — Deletes the session data on the server.
session_regenerate_id — Gives the session a new identifier, keeping the data.