Subscribe to newsletter in WordPress with confirmation emails routes and unsubscribe.

Subscribe-to-newsletter.php

<?php

// Create database table
function wow_create_subscribers_table() {
    global $wpdb;
    $table_name = $wpdb->prefix . 'wow_subscribers';    
    $sql = "CREATE TABLE $table_name (
        id int(11) NOT NULL AUTO_INCREMENT,
        email varchar(100) NOT NULL UNIQUE,
        code varchar(50) NOT NULL UNIQUE,
        active int(11) NOT NULL,
        PRIMARY KEY (id)         
    ) ENGINE=InnoDB DEFAULT CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci;";

    require_once( ABSPATH . 'wp-admin/includes/upgrade.php' );
    dbDelta($sql);
}
add_action('init', 'wow_create_subscribers_table');
add_action('after_setup_theme', 'wow_create_subscribers_table');

// Subscribe user
function wow_subscribe_user(WP_REST_Request $request) {    
    $email = $request->get_param('email');
    $nounce = $request->get_param('nounce');
    // if(wp_verify_nonce($nounce)) {
        // Unique id
        $code = uniqid();
        // Add to db
        $ok = wow_add_subscriber($email, $code);        
        if($ok) {
            // Return json
            return [
                'message' => 'Success',
                'email' => $email,
            ];  
        }
    // }

    return [
        'message' => 'Error',
        'email' => $email,
    ]; 
}

// Is subscriber exists
function wow_exist_subscriber($email) {
    global $wpdb;
    $table_name = $wpdb->prefix . 'wow_subscribers';    
    $row = $wpdb->get_row($wpdb->prepare("SELECT id FROM {$table_name} WHERE email=%s", $email));    
    if($row?->id) {
        return true;
    }
    return false;
}

// Add to db
function wow_add_subscriber($email, $code) {
    global $wpdb;
    $table_name = $wpdb->prefix . 'wow_subscribers';    
    if (filter_var($email, FILTER_VALIDATE_EMAIL)) {
        try {
            // Delete if email exists
            if(wow_exist_subscriber($email)) {
                wow_remove_subscriber($email);
            }
            // Disable duplicate warnings
            // mysqli_report(MYSQLI_REPORT_ERROR | MYSQLI_REPORT_STRICT);
            $ok = $wpdb->insert($table_name, ['email' => $email, 'code' => $code, 'active' => 0], ['%s','%s','%d']);
            if($ok) {            
                wow_send_confirmation_email($email, $code);
                return true;
            }            
        }catch(\Throwable $e) {
            return true;   
        }
        return false;
    }
}

// Remove from db
function wow_remove_subscriber($email) {
    global $wpdb;
    $table_name = $wpdb->prefix . 'wow_subscribers';    
    if (!empty($email)) {
        try {
            $ok = $wpdb->query($wpdb->prepare("DELETE FROM $table_name WHERE email = %s", [$email]));
            if($ok) {            
                return true;
            }
        }catch(\Throwable $e) {
            return true;   
        }
    }    
    return false;
}

// Activate
function wow_update_subscriber($code) {
    global $wpdb;
    $table_name = $wpdb->prefix . 'wow_subscribers';    
    if (!empty($code)) {
        try {
            // Is activated
            $row = $wpdb->get_row($wpdb->prepare("SELECT active FROM {$table_name} WHERE code=%s", $code));
            if($row?->active) {
                return true;
            }
            // Activate
            $ok = $wpdb->query($wpdb->prepare("UPDATE $table_name SET active = 1 WHERE code = %s", [$code]));
            if($ok) {            
                return true;
            }
        }catch(\Throwable $e) {
            return true;   
        }
    }    
    return false;
}

// Send email
function wow_send_confirmation_email($email, $code) {    
    $subject = 'Confirm your email address.';
    $headers = [
        'Content-Type: text/html; charset=UTF-8',
        // 'From: Newsletter <' . get_option('admin_email') . '>',
        // 'Reply-To: ' . get_option('admin_email'), // Client or admin email
    ];		
    // Html body
    $body = '
    <h1>Welcome!</h1>
    <h3>Confirm your email address.</h3>
    </br>
    <a href="' . site_url('/') . 'confirm/email/' . $code . '">Confirm Email</a>
    </br></br></br>
    <a href="' . site_url('/') . 'unsubscribe/email/' . $email . '">Unsubscribe</a>
    ';
    // Get html template
    // ob_start();
    // include get_template_directory() . '/templates/emails/confirm-html.php';
    // $body = ob_get_clean();
    // Send email
    wp_mail($email, $subject, $body, $headers);	
}

// [GET] Subscribe api route
add_action('rest_api_init', function () {    
    // example.org/wp-json/wow/v1/[email protected]&nounce=hash12345
    register_rest_route( 'wow/v1', '/subscribe', array(    
        'methods' => 'GET',
        'callback' => 'wow_subscribe_user',
        'permission_callback' => '__return_true',
    ));    
});

// [GET] Unsubscribe api route
add_action('rest_api_init', function () {    
    // example.org/wp-json/wow/v1/[email protected]
    register_rest_route( 'wow/v1', '/unsubscribe', array(    
        'methods' => 'GET',
        'callback' => 'wow_unsubscribe_user',
        'permission_callback' => '__return_true',
    ));    
});

// [GET] Create confirm route
add_action('init', function() {
    // Flush permalinks from php or go to WP Admin > Settings > Permalinks > Save.
    add_rewrite_rule('confirm/email/([a-z0-9]+)[/]?$', 'index.php?wow_email=$matches[1]', 'top');    
    add_rewrite_rule('unsubscribe/email/([a-z0-9-@.]+)[/]?$', 'index.php?wow_remove_email=$matches[1]', 'top');    
    flush_rewrite_rules();
});

// Update
add_filter('query_vars', function( $query_vars ) {
    $query_vars[] = 'wow_email';
    $query_vars[] = 'wow_remove_email';
    return $query_vars;
});

// Show templates
add_filter('template_include', function( $template ) {
    if (!empty(get_query_var('wow_email'))) {    
        // Get code
        $code = get_query_var('wow_email');
        // Update table with code
        if(wow_update_subscriber($code)) {
            // Success
            return get_template_directory() . '/templates/emails/email-confirm-success.php';
        }
        // Error
        return get_template_directory() . '/templates/emails/email-confirm-error.php';
    }

    if (!empty(get_query_var('wow_remove_email'))) {
        // Get code    
        $email = get_query_var('wow_remove_email');
        // Update table with code
        if(wow_remove_subscriber($email)) {
            // Success
            return get_template_directory() . '/templates/emails/email-unsubscribe-success.php';
        }
        // Error
        return get_template_directory() . '/templates/emails/email-unsubscribe-error.php';
    }

    return $template;
});