PHP包过期日期不正确

this is my script which activates the user one they have paid. For some reason their expire date is really off and random. Here is my script:

This package is only suppose to last 30 days,

Thanks.

<?php

error_reporting(E_ALL);
ini_set('display_errors', 1);

require '../includes/db.php';

if(isset($_POST['item_name'], $_POST['item_number'])) {

    $item_info = explode('_', $_POST['item_number']);
    $packageid = $item_info[0];
    $userid = $item_info[1];

    $sth = $odb->prepare("SELECT * FROM `plans` WHERE `ID` = :id");
    $sth->bindParam(':id', $packageid);
    $sth->execute();

    $data = $sth->fetch(PDO::FETCH_ASSOC);

    $days_to_unixtime = strtotime("{$data['mbt']} days");

    $sth = $odb->prepare("UPDATE `users` SET `membership` = :packageid, `expire` = :expire WHERE `ID` = :userid");
    $sth->bindParam(':packageid', $packageid);
    $sth->bindParam(':expire', $days_to_unixtime);
    $sth->bindParam(':userid', $userid);
    $sth->execute();

    file_put_contents('/tmp/accessed.txt', 'yes');

}

You forgot the + in front of number of days.

strtotime("+ {$data['mbt']} days");