Как сохранить один checkbox в session?

<?php $_SESSION['test'] = isset($_POST['test']); ?>
<form method="POST">
<div>Тест</div>
<input name="test" value="on" type="checkbox" <?php   if ($_SESSION['test'] == "on"){echo 'checked '; }  ?>/> 
<input type="submit" value="Upgrade">
</form>

<?php  $_SESSION['test'] = !empty($_POST['test']); ?>
<form method="POST">
<div>Тест</div>
<input name="test" value="on" type="checkbox" <?php   if ($_SESSION['test'] == "on"){echo 'checked '; }  ?>/> 
<input type="submit" value="Upgrade">
</form>

<?php if (array_key_exists('test', $_POST)) {	
  $_SESSION['test'] = $_POST['test'];
} ?>
<form method="POST">
<div>Тест</div>
<input name="test" value="on" type="checkbox" <?php   if ($_SESSION['test'] == "on"){echo 'checked '; }  ?>/> 
<input type="submit" value="Upgrade">
</form>