为什么在刷新页面时我的PHP会话没有更新？

<?php
session_start();
if (isset($_SESSION['tt'])) {
    echo 'delete session' . $_SESSION['tt'];
    unset($_SESSION['tt']);
    return;
} else {
    echo 'save session';
    $_SESSION['tt'] = '123456';
    return;
}
?>

when I refresh this page, always echo 'save session', then I delete unset($_SESSION['tt']), refresh, always echo 'delete session' but I undo delete , refresh , still echo 'delete session'.

I need echo 'save……' and 'delete……' in turn when I refresh many times, it not work. please help!

Set session.auto_start variable to 1 in php.ini file, some instalation don't load session by default