gistfile1.txt

add_filter('wp_mail', 'handle_wp_mail');
function handle_wp_mail($args) {
  if ($args['subject'] == 'Welcome to YOUR SITE') {
    $wpdb = $GLOBALS['wpdb'];
    $email = $args['to'][0];
    $user = get_user_by('email', $email);
    
    //Generate a new user_activation_key (eg password reset key)
    //This allows us to create a link that a new user can click to set their initial password
    $key = wp_generate_password(20, false);
    require_once(ABSPATH . WPINC . '/class-phpass.php');
    $hasher = new PasswordHash(8, true);
    $hashed = time() . ':' . $hasher->HashPassword($key);
    $wpdb->update($wpdb->users, ['user_activation_key' => $hashed], ['user_login' => $user->user_login]);

    $regUrl = network_site_url("wp-login.php?action=rp&key=$key&login=" . rawurlencode($user->user_login), 'login');
    
    $newMessage = <<<EOT
<h2>Welcome to YOUR SITE</h2>
<p>Dear User,</p>
<p>Thanks for registering!</p>
<p><a href="$regUrl">Click here to set your password.</a></p>
EOT;

    $newMail = [
      'to' => $args['to'],
      'subject' => $args['subject'],
      'headers' => $args['headers'],
      'attachments' => $args['attachments'],
      'message' => $newMessage,
    ];
    
    return $newMail;
  }
}