whmcsTools/accept_order.php

<?php

// this hook sets the order status to accepted after the invoice is fully paid

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

use WHMCS\Database\Capsule;
function get_order($invoiceid) {
    $order = Capsule::table('tblorders')->where('invoiceid', $invoiceid)->first();
    return $order->id;
}

add_hook('InvoicePaid', 1, function($vars) {
    $orderId = get_order($vars['invoiceid']);
    $api = localAPI('acceptorder', array('orderid' => $orderId));
    if ($api['result'] == 'success') {
        return true;
    } else {
        $errormessage .= $api['message'];
        return false;
    }
});

?>