Sends a confirmation request email to a user when they sign up for a new user account (without signing up for a site
at the same time). The user account will not become active until the confirmation link is clicked.

This is the notification function used when no new site has
been requested.

Filter {@see ‘wpmu_signup_user_notification’} to bypass this function or
replace it with your own notification behavior.

Filter {@see ‘wpmu_signup_user_notification_email’} and
{@see ‘wpmu_signup_user_notification_subject’} to change the content
and subject line of the email sent to newly registered users.

function wpmu_signup_user_notification( $user_login, $user_email, $key, $meta = array() ) {
	/**
	 * Filters whether to bypass the email notification for new user sign-up.
	 *
	 * @since MU (3.0.0)
	 *
	 * @param string $user_login User login name.
	 * @param string $user_email User email address.
	 * @param string $key        Activation key created in wpmu_signup_user().
	 * @param array  $meta       Signup meta data. Default empty array.
	 */
	if ( ! apply_filters( 'wpmu_signup_user_notification', $user_login, $user_email, $key, $meta ) ) {
		return false;
	}

	$user            = get_user_by( 'login', $user_login );
	$switched_locale = switch_to_locale( get_user_locale( $user ) );

	// Send email with activation link.
	$admin_email = get_site_option( 'admin_email' );

	if ( '' === $admin_email ) {
		$admin_email = 'support@' . wp_parse_url( network_home_url(), PHP_URL_HOST );
	}

	$from_name       = ( '' !== get_site_option( 'site_name' ) ) ? esc_html( get_site_option( 'site_name' ) ) : 'WordPress';
	$message_headers = "From: "{$from_name}" n" . 'Content-Type: text/plain; charset="' . get_option( 'blog_charset' ) . ""n";
	$message         = sprintf(
		/**
		 * Filters the content of the notification email for new user sign-up.
		 *
		 * Content should be formatted for transmission via wp_mail().
		 *
		 * @since MU (3.0.0)
		 *
		 * @param string $content    Content of the notification email.
		 * @param string $user_login User login name.
		 * @param string $user_email User email address.
		 * @param string $key        Activation key created in wpmu_signup_user().
		 * @param array  $meta       Signup meta data. Default empty array.
		 */
		apply_filters(
			'wpmu_signup_user_notification_email',
			/* translators: New user notification email. %s: Activation URL. */
			__( "To activate your user, please click the following link:nn%snnAfter you activate, you will receive *another email* with your login." ),
			$user_login,
			$user_email,
			$key,
			$meta
		),
		site_url( "wp-activate.php?key=$key" )
	);

	$subject = sprintf(
		/**
		 * Filters the subject of the notification email of new user signup.
		 *
		 * @since MU (3.0.0)
		 *
		 * @param string $subject    Subject of the notification email.
		 * @param string $user_login User login name.
		 * @param string $user_email User email address.
		 * @param string $key        Activation key created in wpmu_signup_user().
		 * @param array  $meta       Signup meta data. Default empty array.
		 */
		apply_filters(
			'wpmu_signup_user_notification_subject',
			/* translators: New user notification email subject. 1: Network title, 2: New user login. */
			_x( '[%1$s] Activate %2$s', 'New user notification email subject' ),
			$user_login,
			$user_email,
			$key,
			$meta
		),
		$from_name,
		$user_login
	);

	wp_mail( $user_email, wp_specialchars_decode( $subject ), $message, $message_headers );

	if ( $switched_locale ) {
		restore_previous_locale();
	}

	return true;
}