symfony_example_app/tests/Controller/IndexControllerTest.php

<?php
declare(strict_types=1);

namespace App\Tests\Controller;

use App\Service\QRCodeProviderInterface;
use Symfony\Bundle\FrameworkBundle\KernelBrowser;
use Symfony\Bundle\FrameworkBundle\Test\WebTestCase;

class IndexControllerTest extends WebTestCase
{
    public function testFormIsPresentAfterLoadHomepage(): void
    {
        $client = static::createClient();
        $client->request('GET', '/');

        $this->assertResponseIsSuccessful();
        $this->assertSelectorExists('form');
    }

    public function testImageIsPresentAfterSubmitForm(): void {
        $client = $this->generateClientForSubmitTest();
        $response = $client->request('GET', '/');
        $this->assertResponseIsSuccessful();
        $form = $response->selectButton('Vygenerovat QR kód')->form();
        $dt = new \DateTime('now+1 hour'); // In case of 23:59:59
        $client->submit($form, [
            'qr_code' => [
                    'iban' => 'CZ5508000000001234567899',
                    'dueDate' => $dt->format('Y-m-d'),
                    'message' => 'foo',
                    'money' => [
                        'currency' => 'CZK',
                        'amount' => '100'
                    ],
                    'paymentIdentification' => [
                        'variableSymbol' => '123',
                        'constantSymbol' => '123',
                        'specificSymbol' => '123'
                    ]
                ]
        ]);

        $this->assertResponseIsSuccessful();
        $this->assertSelectorExists('form');
        $this->assertSelectorExists('img');
    }

    public function testImageIsNotPresentAfterFailSubmitForm(): void {
        $client = $this->generateClientForSubmitTest();
        $response = $client->request('GET', '/');
        $this->assertResponseIsSuccessful();
        $form = $response->selectButton('Vygenerovat QR kód')->form();
        $dt = new \DateTime('now+1 hour'); // In case of 23:59:59
        $client->submit($form, [
            'qr_code' => [
                'iban' => 'IBAN_THAT_NOT_EXISTS',
                'dueDate' => $dt->format('Y-m-d'),
                'message' => 'foo',
                'money' => [
                    'currency' => 'CZK',
                    'amount' => '100'
                ],
                'paymentIdentification' => [
                    'variableSymbol' => '123',
                    'constantSymbol' => '123',
                    'specificSymbol' => '123'
                ]
            ]
        ]);

        $this->assertResponseIsUnprocessable();
        $this->assertSelectorExists('form');
        $this->assertSelectorNotExists('img');
    }

    private function generateClientForSubmitTest(): KernelBrowser
    {
        $mock = $this->createMock(QRCodeProviderInterface::class);
        $mock->expects($this->any())
            ->method('generateQRCodeFromEntity')
            ->will($this->returnValue('foo'));


        $client = static::createClient();
        $client->disableReboot();

        self::getContainer()->set('App\Service\QRCodeProviderInterface', $mock);

        return $client;
    }
}