request-list.php

<?php
// Add product to request list
function add_product_id($product_id) {
    $product_ids = get_product_ids();

    // If product id is in array, return
    if (in_array($product_id, $product_ids)) {
        return;
    }

    // Add product id to array
    array_push($product_ids, intval($product_id));

    // Set cookie with updated array
    setcookie("productIDs", serialize($product_ids), time() + (3600 * 24 * 7), '/');
}

// Remove product from request list
function remove_product_id($product_id) {
    $product_ids = get_product_ids();

    // If product id is in array, return
    if (!in_array($product_id, $product_ids)) {
        return;
    }

    // Remove product id from array
    $product_ids = array_diff($product_ids, array(intval($product_id)));

    // Set cookie with updated array
    setcookie("productIDs", serialize($product_ids), time() + (3600 * 24 * 7), '/');
}

// Remove product from request list
function clear_product_ids() {
    // Set product ids to an empty array
    $product_ids = [];

    // Set cookie with updated array
    setcookie("productIDs", serialize($product_ids), time() + (3600 * 24 * 7), '/');
}

// Return product request list
function get_product_ids() {
    // Convert serialized string back to array
    $product_ids = unserialize($_COOKIE["productIDs"]);

    // If productIDs is not an array, return an empty array
    if (!is_array($product_ids)) {
        return [];
    }

    return $product_ids;
}

// Checks if current product id is in the product_ids array
function check_product_id($product_id) {
    return in_array($product_id, get_product_ids());
}

// Returns number of products in product list
function get_request_list_count() {
    return count(get_product_ids());
}

function request_redirect() {
    if (isset($_GET['redirect'])) {
        wp_redirect(get_permalink($_GET['redirect']));
    }
}

// Update request list
function request_list_update() {
    // Add
    if (isset($_GET['add'])) {
        add_product_id($_GET['add']);
        request_redirect();
        exit;
    }

    // Remove
    if (isset($_GET['remove'])) {
        remove_product_id($_GET['remove']);
        request_redirect();
        exit;
    }

    // Clear
    if (isset($_GET['clear']) && $_GET['clear'] == '1') {
        clear_product_ids();
        request_redirect();
        exit;
    }
}
add_action('init', 'request_list_update');