// tests/AppBundle/Controller/PostControllerTest.php
namespace Tests\AppBundle\Controller;

use Symfony\Bundle\FrameworkBundle\Test\WebTestCase;

class PostControllerTest extends WebTestCase
{
    public function testShowPost()
    {
        $mock = /** mocked version of the service with data relevant to this test**/
        $client = static::createClient(['app.externalapi' => $mock);

        $crawler = $client->request('GET', '/post/hello-world');

        $this->assertGreaterThan(
            0,
            $crawler->filter('html:contains("Hello World")')->count()
        );
    }
}