auth.php

<?php
/**
 * Auto Login Hash Merge Tag Email Template ( Versão resumida )
 * @author     Mauricio Gofas | gofas.net
 * @copyright  Copyright (c) 2016 https://gofas.net
 */
 
 // Modelo de URL para colar no template de email:
// {$whmcs_url}auth.php?email={$client_email}&hash={$hash}&whmcsurl={$whmcs_url}&goto=viewinvoice.php?id={$invoice_id}
$autoauthkey = "xxxxx_autoauthkey_xxxxx"; // chave igual à inserida no arquivo /configuration.php
$secret_key =  "xxxxx_secret_key_xxxx"; // chave igual à inserida no arquivo /includes/hooks/gofas_hash_email.php
$whmcsurl = $_GET["whmcsurl"].'dologin.php';
if (md5($_GET['email'].$secret_key) != $_GET['hash']){ // Confere se o hash é válido
die('Hash inválido. Admin: verifique o formato da tag do template de e-mail, as credenciais e os dados do cliente.'); // Morre se o hash é Inválido
} 
// Continua a execução do script e permite o acesso se o hash é válido 
$email		= $_GET['email'];
$timestamp	= time();
$goto		= $_GET["goto"];
$hash		= sha1($email.$timestamp.$autoauthkey);
$url		= $whmcsurl."?email=$email&timestamp=$timestamp&hash=$hash&goto=".urlencode($goto);
header("Location: $url");
exit;
?>

gofas_hash_email.php

<?php
/**
 * Auto Login Hash Merge Tag Email Template ( Versão resumida )
 * @author     Mauricio Gofas | gofas.net
 * @copyright  Copyright (c) 2016 https://gofas.net
 */
if (!defined('WHMCS')) die('This file cannot be accessed directly');
function gofas_hash_email_template($vars) {
	$whmcsAdmin							= 1;
	$invoiceID							= $vars['relid'];
 	$getinvoice							= 'getinvoice';
	$getinvoiceid['invoiceid']			= $invoiceID;
	$GetInvoiceResults					= localAPI($getinvoice,$getinvoiceid,$whmcsAdmin);
	$userID								= $GetInvoiceResults['userid'];
 	$getClient							= 'getclientsdetails';
	$getClientValues['clientid']		= $userID;
	$getClientValues['stats']			= true;
	$getClientValues['responsetype']	= 'json';
	$getClientResults					= localAPI($getClient,$getClientValues,$whmcsAdmin);
	$secret_key							= 'xxxxx_secret_key_xxxx'; // chave igual à inserida no /auth.php
	$merge_fields = array();
	$merge_fields['hash']				= md5($getClientResults['client']['email'].$secret_key);
	return $merge_fields;
}
add_hook('EmailPreSend',1,'gofas_hash_email_template');
?>