View file ProForum/Files/application/app/Http/Controllers/Gateway/PaypalSdk/Core/SandboxEnvironment.php

<?php

namespace App\Http\Controllers\Gateway\PaypalSdk\Core;

class SandboxEnvironment extends PayPalEnvironment
{
    public function __construct($clientId, $clientSecret)
    {
        parent::__construct($clientId, $clientSecret);
    }

    public function baseUrl()
    {
        return "https://api.sandbox.paypal.com";
    }
}