whmcsTools/get_user_products.php

<?php

// returns an array of product ids already bought be the client

if (!defined("WHMCS"))
    die("This file cannot be accessed directly");

add_hook('ClientAreaPageCart', 1, function($vars) {
    // check client login status
    if(!isset($vars['clientsdetails']['id'])) {
        return;
    }
    $active_pids = array();
    $clientId = $vars['clientsdetails']['id'];
    // get client products
    $clientProducts = localAPI('GetClientsProducts', array('clientid' => $clientId, 'stats' => false));
    if(!$clientProducts['result'] == 'success'){
        return;
    }
    foreach($clientProducts['products']['product'] as $product) {
        array_push($active_pids, $product['pid']);
    }
    return array('clientPids' => $active_pids);
});