为什么我在尝试使用phpmailer发送电子邮件激活新用户帐户时收到此错误

Question

i am trying to understand why my subject method in index.php triggers an error of not being defined.i am using phpmailer 5.2.7 with php 7.2 and wampserver 3.1.7

//here is my extended class from phpmailer//

<?php
include('phpmailer.php');

class Mail extends PhpMailer
{

    // Set default variables for all new objects

    public $From     = '[e-mail]';
    public $FromName =  MM;
    public $Host     = 'smtp.gmail.com';
    public $Mailer   = 'smtp';
    public $SMTPAuth =  true;
    public $Username = '[e-mail]';
    public $Password = 'xxxxxx';
    public $SMTPSecure = 'ssl';
    public $WordWrap = 75;



    public function subject($subject)
    {
        $this->Subject = $subject;
    }

    public function body($body)
    {
        $this->Body = $body;
    }

    public function send()
    {
        $this->AltBody = strip_tags(stripslashes($this->Body))."

";
        $this->AltBody = str_replace("&nbsp;", "

", $this->AltBody);
        return parent::send();
    }
}

and here is part of my index page where i have defined my variables

$to = $_POST['email'];
            $subject = "Registration Confirmation";
            $body = "<p>Thank you for registering at demo site.</p>
            <p>To activate your account, please click on this link: <a href='".DIR."activate.php?x=$id&y=$activasion'>".DIR."activate.php?x=$id&y=$activasion</a></p>
            <p>Regards Site Admin</p>";

            $mail = new PHPMailer(true);
            $mail->setFrom(SITEEMAIL);
            $mail->addAddress($to);
            $mail->subject($subject);
            $mail->body($body);
            $mail->send();

            //redirect to index page
            header('Location: index.php?action=joined');
            exit;