PHP how to check the status of a checkbox (switch) after the submit

in a form i put in a checkbox (switch), how do i post after submitting to checking the status of the switch for MySQL update? thank you

Consequently the MySQL table does not change.

From checkbox and submit

<label class="mdl-switch mdl-js-switch mdl-js-ripple-effect" for="switch-1">
<input type="checkbox" id="switch-1" class="mdl-switch__input" checked>
<span class="mdl-switch__label">Autenticazione a due fattori</span>
</label>
....
<input onclick="conferma(); update_switch()" class="mdl-button mdl-js-button 
mdl-button--raised mdl-js-ripple-effect mdl-button--accent" type="submit" 
value="Salva">
</form>

Switch control and MySQL update

<script>
    function update_switch() {
        <?php 
        session_start();
        mysql_connect(localhost) or die(mysql_error()); 
        mysql_select_db("*******") or die(mysql_error()); 
        $user = $_SESSION['users'];
        if(!isset($_POST['switch-1'])){                       
        $sql = "UPDATE `********`.`login_users` SET `auth` = \'checked\' 
        WHERE username = '$user'";
        } else {
        $sql = "UPDATE `*******`.`login_users` SET `auth` = \'no\' WHERE 
        username = '$user'";
        }

        ?>
        }
        update_switch();
</script>