问如何在mysql数据库中存储注销时间？

EN

<?php 
session_start();

// variable declaration
$username = "";
$email    = "";
$errors = array(); 
$_SESSION['success'] = "";



// connect to database
$db = mysqli_connect('localhost', 'iotaeyjx_login', 'amitsaha1993', 'iotaeyjx_login');

// REGISTER USER
if (isset($_POST['reg_user'])) {
    // receive all input values from the form
    $username = mysqli_real_escape_string($db, $_POST['username']);
    $email = mysqli_real_escape_string($db, $_POST['email']);
    $password_1 = mysqli_real_escape_string($db, $_POST['password_1']);
    $password_2 = mysqli_real_escape_string($db, $_POST['password_2']);

    // form validation: ensure that the form is correctly filled
    if (empty($username)) { array_push($errors, "Username is required"); }
    if (empty($email)) { array_push($errors, "Email is required"); }
    if (empty($password_1)) { array_push($errors, "Password is required"); }

    if ($password_1 != $password_2) {
        array_push($errors, "The two passwords do not match");
    }

    // register user if there are no errors in the form
    if (count($errors) == 0) {
        $password = md5($password_1);//encrypt the password before saving in the database
        $query = "INSERT INTO users (username, email, password) 
                  VALUES('$username', '$email', '$password')";
        mysqli_query($db, $query);

        $_SESSION['username'] = $username;
        $_SESSION['success'] = "You are now logged in";
        header('location: index.php');
    }

}

// ... 

// LOGIN USER
if (isset($_POST['login_user'])) {
    $username = mysqli_real_escape_string($db, $_POST['username']);
    $password = mysqli_real_escape_string($db, $_POST['password']);

    if (empty($username)) {
        array_push($errors, "Username is required");
    }
    if (empty($password)) {
        array_push($errors, "Password is required");
    }

    if (count($errors) == 0) {
        $password = md5($password);
        $query = "SELECT * FROM users WHERE username='$username' AND password='$password'";


        $results = mysqli_query($db, $query);


        if (mysqli_num_rows($results) == 1) {

            $sql_2 = "INSERT INTO login_logs(username) VALUES ('" . $username . "')";
            $results2 = mysqli_query($db, $sql_2);



            $_SESSION['username'] = $username;
            $_SESSION['success'] = "You are now logged in";
            header('location: index.php');
        }else {
            array_push($errors, "Wrong username/password combination");
        }
    }
}

if (isset($_POST['logout_user'])) {


            $sql_3 = "UPDATE login_logs SET logout_time = current_timestamp where username ='$username'";
           $results3 = mysqli_query($db, $sql_3);


           $_SESSION['success'] = "You are now logged out";
           header('location: index.php');

}

<?php 



session_start(); 

if (!isset($_SESSION['username'])) {
    $_SESSION['msg'] = "You must log in first";
    header('location: login.php');
}

if (isset($_GET['logout'])) {
    session_destroy();
    unset($_SESSION['username']);
    header("location: login.php");
}

    <!-- notification message -->
    <?php if (isset($_SESSION['success'])) : ?>
        <div class="error success" >
            <h3>
                <?php 
                    echo $_SESSION['success']; 
                    unset($_SESSION['success']);
                ?>
            </h3>
        </div>
    <?php endif ?>

    <!-- logged in user information -->
    <?php  if (isset($_SESSION['username'])) : ?>
        <p>Welcome <strong><?php echo $_SESSION['username']; ?></strong></p>

        <br>
        <p> <a href="index.php?logout='1'" style="color: red;">Are you Ready to go HOME? Click Here!!</a> </p>
    <?php endif ?>


</div>