在过期日期前自动发送电子邮件

在我的应用程序中，我需要电子邮件自动发送到注册客户端，因为会员资格即将到期，是cron作业是自动发送邮件的唯一解决方案，我尝试了cron作业，在那里我能够按预定时间发送邮件，但我将如何采取客户端各自的到期日期并触发邮件

    <?php
// Set this to your timezone
date_default_timezone_set('asia/kolkata');
// Start at 8:00 AM (24-hour time)
$startTime = mktime(8, 0, 0);
// End at 5:00 PM (24-hour time)
$endTime = mktime(17, 0, 0);

$currentTime = time();
// Do not send the email if it is outside of the allowed hours
if($currentTime < $startTime || $currentTime > $endTime)
{
    print('Not sending an email after hours.');
    die();
}
// Get the current day of the week as an index (0=Sunday, 6=Saturday)
$dayOfWeek = date('w');
// Do not send the email on weekends
if($dayOfWeek == 0 || $dayOfWeek == 6)
{
    print('Not sending an email on the weekends.');
    die();
}
// Info of person to receive the tests
define('TO_EMAIL',      'ramya.krish7@gmail.com');
define('TO_NAME',       'ramya');
// Info of person sending the tests
define('FROM_EMAIL',    'webmaster@serversendingtests.com');
define('FROM_NAME', 'Email Tester');
// Example: 8:00 am on 1 Nov 2010
$subject = 'Test: ' . date('g:i a 'o'n j M Y');
$message = 'This email was automatically generated. Please send an email to yourusername@youremailprovider.com if you would like to disable these automated tests.';
$result = mail(TO_NAME . ' <' . TO_EMAIL . '>', $subject, $message, 'From: ' . FROM_NAME . ' <' . FROM_EMAIL . '>');
var_dump($result)