How to add a custom class to the current menu item

<?php
add_filter('nav_menu_css_class', 'add_class_to_active_menu_item', 10, 2);

/**
 * Method add_class_to_active_menu_item
 *
 * This method adds an extra class to the menu item when it's active.
 * Should be called in the hook 'nav_menu_css_class'.
 *
 * https://developer.wordpress.org/reference/hooks/nav_menu_css_class/
 *
 * @param array   $classes Array of the CSS classes applied to the menu item's <li> element.
 * @param WP_Post $item    The current menu item object.
 *
 * @return array Contains classes
 */
function add_class_to_active_menu_item($classes, $item) {

    $full_request_url = '';

    if (!empty($_SERVER['HTTPS']) && $_SERVER['HTTPS'] !== 'off') {
        $full_request_url .= 'https://';
    } else {
        $full_request_url .= 'http://';
    }

    $full_request_url .= $_SERVER['HTTP_HOST'] . $_SERVER['REQUEST_URI'];

    if ($item->post_type == 'nav_menu_item' && $item->url == $full_request_url) {
        $classes[] = 'current-menu-item';
    } else {
        return $classes;
    }

    return array_unique($classes);
}