试试这个：

// Update the membership level for the user
$level_id = substr($decrypted_data, 19, 1);
pmpro_changeMembershipLevel($level_id, $user_id); // Change membership level

// Get the level details to calculate the End Date
$level = pmpro_getLevel($level_id);
if ($level->expiration_number > 0) {
    $end_date = date('Y-m-d', strtotime("+{$level->expiration_number} {$level->expiration_period}", current_time('timestamp')));
} else {
    $end_date = '0000-00-00 00:00:00';
}

if (!$order->payment_transaction_id) {

    // Query to check if order_id/payment_transaction_id already exists in database, in the event someone is trying to reuse a successful payment ID

    $order->code = $order_id;
    $order->user_id = $user_id;
    $order->payment_transaction_id = $order_id;
    $order->total = $membership_amount;
    $order->membership_id = $level_id;
    $order->gateway = 'PayStack';
    $order->status = 'success';
    $order->enddate = $end_date; // Set the End Date for the order
    // Save object
    $order->saveOrder();
}

它使用pmpro_getLevel（$level_id）检索成员资格级别详细信息，然后根据到期号和期限计算EndDate。

@诺埃尔Bürgler感谢您的回答，但它不能工作，因为MemberOrder类没有enddate属性。然而，经过一番折腾，我能够解决这个问题，我决定在这里分享答案，因为我注意到很多人面临同样的问题，不想为此支付PMPRO支持。
下面是我如何解决这个问题的：

// First, establish a database connection
$servername = "localhost";
$username = "username";
$password = "password";
$dbname = "database_name";

$conn = new mysqli($servername, $username, $password, $dbname)

$plan_id = 1; //set your plan id

$expiration_date = date('Y-m-d', strtotime('+30 days')); //set your enddate

pmpro_changeMembershipLevel($plan_id, $user_id); //update user membership level

$order = new MemberOrder();

$order->code = $decoded_data->data->id;
$order->user_id = $user_id;
$order->payment_transaction_id = $decoded_data->data->reference;
$order->total = $decoded_data->data->amount / 100;
$order->membership_id = $plan_id;
$order->gateway = 'PayStack';
$order->status = 'success';

// Save object
$order->saveOrder(); //save order

// Next, define the update query to update enddate
$sql = "UPDATE wp_pmpro_memberships_users SET enddate = '$expiration_date' WHERE user_id = '$user_id'";

// Execute the enddate update query
$conn->query($sql);

// close the database connection
$conn->close();

希望这能帮助其他和我一样经历同样问题的人：）