PHP how to know user has read the email? | PHP

for($n=0; $nIsHTML(true);
$mail->Subject = $subject;
$function = "";
$bodyOpen = "";
$msg_body .= $body .= "

Hello Everyone

";
$mail->Body = $function.$bodyOpen.$msg_body;
$mail->WordWrap = 50;
$mail->FromName = 'Muhammad Sajid';
$mail->IsMAIL();
$mail->From = 'webspot49@gmail.com';
$mail->AddAddress($checkBox[$n]);
$sent = $mail->Send();
}