"Email Subscribers & Newsletters" WordPress Plugin - Add ability to subscribe/unsubscribe from backend

functions.php

<?php
 /* Email Subscribers & Newsletters - by Icegram (https://wordpress.org/plugins/email-subscribers)
 These functions allow you to subscribe/unsubscribe a user to an email group without using a signup form. */

/**
 * Subscribe a user to a given newsletter group
 *
 * @param $user_id
 * @param $group
 * @throws Exception
 * @return bool
 */
function subscribe_user_to_email_notifications( $user_id, $group ) {
	if ( empty( $group ) ) {
		throw new InvalidArgumentException( 'Email group is required' );
	}
	$user = get_userdata( $user_id );
	if ( !$user ) {
		throw new Exception( 'User not found' . $user_id );
	}

	$data = array();
	$data['es_form_submit'] = 'yes';
	$data['es_nonce'] = wp_create_nonce( 'es-subscribe' );
	$data['es_email_mail'] = $user->user_email;
	$data['es_email_name'] = $user->first_name;
	$data['es_email_group'] = $group;
	$data['es_email_status'] = 'Confirmed';

	$result = es_cls_dbquery::es_view_subscriber_ins( $data );

	if ($result === 'ext') {
		throw new Exception('Subscription already exists for ' . $user->user_email . ' in the ' . $group . ' group');
	}
	if ($result === 'invalid') {
		throw new Exception('Invalid email address: ' . $user->user_email );
	}

	// Successful subscription
	if ($result === 'sus') {
		return true;
	}
}

/**
 * Get a subscriber record from the database that matches the user and group
 *
 * @param $user_id
 * @param $group
 * @throws Exception
 * @return array|null|object
 */
function get_subscriber( $user_id, $group ) {
	if ( empty( $group ) ) {
		throw new InvalidArgumentException( 'Email group is required' );
	}
	$user = get_userdata($user_id);
	if (!$user) {
		throw new Exception('Invalid user ID');
	}
	$subscriber = es_cls_dbquery::es_view_subscriber_one($user->user_email, $group);
	if (empty($subscriber[0])) {
		throw new Exception('No subscriber found');
	}

	return $subscriber;
}

/**
 * Unsubscribe a user from email notifications
 *
 * @param $user_id
 * @param $group
 * @throws Exception
 * @return bool
 */
function unsubscribe_user_from_email_notifications( $user_id, $group ) {
	$subscriber = get_subscriber( $user_id, $group );
	$subscriber_id = !empty($subscriber[0]['es_email_id']) ? $subscriber[0]['es_email_id'] : 0;

	return es_cls_dbquery::es_view_subscriber_delete($subscriber_id);
}