Web Features Learning Mode

What is the output of this script the third time it is loaded in a browser by the same user?

<?php
    session_start();

    if (!array_key_exists('counter', $_SESSION)) {
        $_SESSION['counter'] = 0;
    }
    else {
        $_SESSION['counter']++;
    }

    session_regenerate_id();

    echo $_SESSION['counter'];
?>
Previous