/** @test */
 function a_customer_is_gold_if_they_have_the_respective_type()
 {
     $specification = new CustomerIsGold();
     $goldCustomer = new Customer('gold');
     $silverCustomer = new Customer('silver');
     $this->assertTrue($specification->isSatisfiedBy($goldCustomer));
     $this->assertFalse($specification->isSatisfiedBy($silverCustomer));
 }
<?php

use App\Specification\Customer;
use App\Specification\CustomerIsGold;
require 'vendor/autoload.php';
$spec = new CustomerIsGold();
var_dump($spec->isSatisfiedBy(new Customer('gold')));