+https://stackoverflow.com/questions/39763567/make-link-expire-after-x-minutes

Send a timestamp in link and on other end while accepting the link I have to check the time with current time stamp.

$base_url='http://172.16.0.60/WebServices/';
$activation=md5($email.time());
$time = time();
email format to HTML
        $mail->Subject = 'Activate Your Account';
        $mail->Body    = 'Hi, <br/> <br/> We need to make sure you are human. Please verify your email and get started using your Website account. <br/> <br/> <a href="'.$base_url.'activation/'.$activation.'">'.$base_url.'activation/'.$activation.$time.'</a>';

While when user clicks on link, following code will check the expiration of link.

$u_time = $this->input->get('time'); // fetching time variable from URL
$cur_time = time(); //fetching current time to check with GET variable's time
if ($cur_time - $u_time < 10800) 
{
 // link is not expired
}
else
{
 // link has been expired
}