how to disable the email message link after first click
<p>I developed a php email activation code for activate user's account after registration, what I want is that if the user click the activation link again after first time, the link will be disabled, any one could help, thanks a lot!
here is my php code:</p>

<pre><code>  $to  =  $email;
  $subject = " Your Registration";
  $message = "Welcome to our website!\r\rThanks for completing registration at www.example.com.
   You can complete registration by clicking the following    link:\rhttp://www.example.com/verify.php?$activationKey\r\rIf this is an error, ignore this    email and you will be removed from our mailing list.\r\rRegards,\ www.example.com Team";
    $headers = 'From: noreply@ example.com' . "\r\n" .

'Reply-To: noreply@ example.com' . "\r\n" .

'X-Mailer: PHP/' . phpversion();

mail($to, $subject, $message, $headers);
</code></pre>
