Place this hook script in the includes/hooks/ directory of the WHMCS folder within your server's webroot to display each client's available credit balance as a sidebar item within the WHMCS Client Area homepage and billing-related pages. Created by combining elements from two scripts from the WHMCS forums and fixing several issues with both. Tes…

creditBalanceSidebar.php

<?php

/**
 * Display Client's Credit Balance as a Sidebar Item in Client Area Homepage and Billing-related Pages
 *
 * @author Josh Wieder
 * @link https://gist.github.com/jwieder/3d470a4e85e041ca41bc2cee0c5aa7a8#file-creditbalancesidebar-php
 * @since WHMCS v6.0.0+
 */

use WHMCS\View\Menu\Item as MenuItem;

# Add Balance To Sidebar
add_hook('ClientAreaSecondarySidebar', 1, function(MenuItem $primarySidebar){
    $filename = APP::getCurrentFileName();
    $client = Menu::context("client");
    $clientid = intval( $client->id );
    $action = $_GET['action'];
    $allowed = array('invoices', 'quotes', 'masspay', 'addfunds');

/* prevents balance display to unauth'd users */
    if ($filename!=='clientarea' || $clientid===0 || strpos($_SERVER['REQUEST_URI'], 'verificationId') !== false || is_null($client)) {
        return;
    }

/*
* uncomment this to hide the sidebar if the client has no balance
* if ($client->credit <= 0.00) { return; } */

    $primarySidebar->addChild('Client-Balance', array(
        'label' => Lang::trans('availcreditbal'),
        'uri' => '#',
        'order' => '1',
        'icon' => 'fa-money'
    ));

    # Get Currency
    $getCurrency = getCurrency($clientid);
    $balanceDisplay = formatCurrency($client->credit, $getCurrency);

    # Retrieve the panel we just created.
    $balancePanel = $primarySidebar->getChild('Client-Balance');

    // Move the panel to the end of the sorting order so it's always displayed
    // as the last panel in the sidebar.
    $balancePanel->moveToBack();
    $balancePanel->setOrder(0);

    # Add Balance.
    $balancePanel->addChild('balance-amount', array(
        'uri' => 'clientarea.php?action=addfunds',
        'label' => '<h4 style="text-align:center;">'.$balanceDisplay.'</h4>',
        'order' => 1
    ));

    $balancePanel->setFooterHtml(
        '<a href="clientarea.php?action=addfunds" class="btn btn-success btn-sm btn-block">
            <i class="fa fa-plus"></i> Add Funds </a>'
    );
});