Require email verification for custom Wordpress registration

functions.php

<?php
// Generate an email verification code and send it to their email
add_action('user_register', 'send_new_user_email_verification', 10, 1);
function send_new_user_email_verification($user_id) {
  $user = get_userdata($user_id);
  $email = $user->user_email;
  $site_name = get_bloginfo('name');
  $code = wp_generate_password(32, false, false);
  $hash = md5($code);
  $link = add_query_arg(array(
    'email-verification' => $code,
    'user'               => $user_id
  ), network_home_url());
  $message = "Please verify your email address by visiting the following link:\n\n$link\n\nThank you.";
  $subject = "$site_name Email Verification";

  add_user_meta($user_id, 'email_verification_hash', $hash);
  wp_mail($email, $subject, $message);
}

// Verify the email verification code when the user visits the link in their email
add_action('init', 'verify_new_user_email_verification', 10, 0);
function verify_new_user_email_verification() {
  if (isset($_GET['email-verification']) && isset($_GET['user'])) {
    $code = $_GET['email-verification'];
    $user_id = $_GET['user'];
    $hash = get_user_meta($user_id, 'email_verification_hash', true);

    if (md5($code) === $hash) {
      delete_user_meta($user_id, 'email_verification_hash');
      wp_safe_redirect(add_query_arg('email-verified', '1', wp_login_url()));
    }
  }
}

// If the user hasn't verified their email, don't allow them to login, generate and send them a new code instead
add_filter('authenticate', 'require_new_user_email_verification', 99, 1);
function require_new_user_email_verification($user) {
  if (is_a($user, 'WP_User')) {
    $user_id = $user->ID;
    $email_unverified = get_user_meta($user_id, 'email_verification_hash', true);

    if ($email_unverified) {
      delete_user_meta($user_id, 'email_verification_hash');
      send_new_user_email_verification($user_id);
      wp_safe_redirect(add_query_arg('email-verification-sent', '1', wp_login_url()));
      exit;
    }
  }

  return $user;
}

// Displays a message on the login screen confirming that the users email address has been verified
// or letting them know that an email has been sent for them to verify their email address
add_filter('login_message', 'email_verification_messages', 10, 1);
function email_verification_messages($message) {
  if (isset($_GET['email-verified'])) {
    $message = 'Your email address has been verified. You may now log in.';
  } elseif (isset($_GET['email-verification-sent'])) {
    $message = 'Please check your inbox for an email containing a link to verify your email address.';
  }

  return $message;
}
?>