Notifies a user that their account activation has been successful.

Filter {@see ‘wpmu_welcome_user_notification’} to disable or bypass.

Filter {@see ‘update_welcome_user_email’} and {@see ‘update_welcome_user_subject’} to
modify the content and subject line of the notification email.

function wpmu_welcome_user_notification( $user_id, $password, $meta = array() ) {
	$current_network = get_network();

	/**
	 * Filters whether to bypass the welcome email after user activation.
	 *
	 * Returning false disables the welcome email.
	 *
	 * @since MU (3.0.0)
	 *
	 * @param int    $user_id  User ID.
	 * @param string $password User password.
	 * @param array  $meta     Signup meta data. Default empty array.
	 */
	if ( ! apply_filters( 'wpmu_welcome_user_notification', $user_id, $password, $meta ) ) {
		return false;
	}

	$welcome_email = get_site_option( 'welcome_user_email' );

	$user = get_userdata( $user_id );

	$switched_locale = switch_to_locale( get_user_locale( $user ) );

	/**
	 * Filters the content of the welcome email after user activation.
	 *
	 * Content should be formatted for transmission via wp_mail().
	 *
	 * @since MU (3.0.0)
	 *
	 * @param string $welcome_email The message body of the account activation success email.
	 * @param int    $user_id       User ID.
	 * @param string $password      User password.
	 * @param array  $meta          Signup meta data. Default empty array.
	 */
	$welcome_email = apply_filters( 'update_welcome_user_email', $welcome_email, $user_id, $password, $meta );
	$welcome_email = str_replace( 'SITE_NAME', $current_network->site_name, $welcome_email );
	$welcome_email = str_replace( 'USERNAME', $user->user_login, $welcome_email );
	$welcome_email = str_replace( 'PASSWORD', $password, $welcome_email );
	$welcome_email = str_replace( 'LOGINLINK', wp_login_url(), $welcome_email );

	$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         = $welcome_email;

	if ( empty( $current_network->site_name ) ) {
		$current_network->site_name = 'WordPress';
	}

	/* translators: New user notification email subject. 1: Network title, 2: New user login. */
	$subject = __( 'New %1$s User: %2$s' );

	/**
	 * Filters the subject of the welcome email after user activation.
	 *
	 * @since MU (3.0.0)
	 *
	 * @param string $subject Subject of the email.
	 */
	$subject = apply_filters( 'update_welcome_user_subject', sprintf( $subject, $current_network->site_name, $user->user_login ) );

	wp_mail( $user->user_email, wp_specialchars_decode( $subject ), $message, $message_headers );

	if ( $switched_locale ) {
		restore_previous_locale();
	}

	return true;
}