Symfony PHPUnit : Failed to start session because header have already been sent

Versions :

• PHP : 8.1
• PHPUnit : 9.5.21
• Symfony 6.1

When the following test is run by PhpUnit, I get an error about the session not being able to be started. Does anyone have an idea of the problem and how to solve it?

The following topics do not answer my problem or the proposed solution doesn't work for me :

• How to fix headers already sent
• Cannot send session cookie headers already sent
• Github discution 43950

I tried the proposed solutions of the second link : @session_start(), @runInSeparateProcess but nothing worked. Maybe I just misunderstand my problem but I'm stuck for a week now.

protected function setUp(): void
{
  @session_start();
  parent::setUp();
}

/**
 * @runInSeparateProcess
 */
public function testLoginFailure(): void
{
    $client = static::createClient();
    $crawler = $client->request('GET', '/login');

    $form = $crawler->selectButton('Login')->form();

    $form['email']->setValue('[email protected]');
    $form['password']->setValue('123abcABC%');

    $crawler = $client->submit($form);
    $this->assertResponseIsSuccessful();
}

<!-- Failed to start the session because headers have already been sent by &quot;C:\Users\cimba\Documents\project\vendor\phpunit\phpunit\src\Util\Printer.php&quot; at line 104. (500 Internal Server Error) -->

C:\Users\cimba\Documents\project\vendor\symfony\framework-bundle\Test\BrowserKitAssertionsTrait.php:142
C:\Users\cimba\Documents\project\vendor\symfony\framework-bundle\Test\BrowserKitAssertionsTrait.php:33
C:\Users\cimba\Documents\project\tests\InternalLoginTest.php:51
C:\Users\cimba\Documents\project\bin\phpunit:11

Caused by
ErrorException: Failed to start the session because headers have already been sent by "C:\Users\cimba\Documents\project\vendor\phpunit\phpunit\src\Util\Printer.php" at line 104. in C:\Users\cimba\Documents\project\vendor\symfony\http-foundation\Session\Storage\NativeSessionStorage.php:135
Stack trace:
#0 C:\Users\cimba\Documents\project\vendor\symfony\framework-bundle\Test\BrowserKitAssertionsTrait.php(33): Symfony\Bundle\FrameworkBundle\Test\WebTestCase::assertThatForResponse(Object(Symfony\Component\HttpFoundation\Test\Constraint\ResponseIsSuccessful), '')
#1 C:\Users\cimba\Documents\project\tests\InternalLoginTest.php(51): Symfony\Bundle\FrameworkBundle\Test\WebTestCase::assertResponseIsSuccessful()
#2 C:\Users\cimba\Documents\project\vendor\phpunit\phpunit\src\Framework\TestCase.php(1545): App\Tests\InternalLoginTest->testLoginFailure()
#3 C:\Users\cimba\Documents\project\vendor\phpunit\phpunit\src\Framework\TestCase.php(1151): PHPUnit\Framework\TestCase->runTest()
#4 C:\Users\cimba\Documents\project\vendor\phpunit\phpunit\src\Framework\TestResult.php(726): PHPUnit\Framework\TestCase->runBare()
#5 C:\Users\cimba\Documents\project\vendor\phpunit\phpunit\src\Framework\TestCase.php(903): PHPUnit\Framework\TestResult->run(Object(App\Tests\InternalLoginTest))
#6 C:\Users\cimba\Documents\project\vendor\phpunit\phpunit\src\Framework\TestSuite.php(670): PHPUnit\Framework\TestCase->run(Object(PHPUnit\Framework\TestResult))
#7 C:\Users\cimba\Documents\project\vendor\phpunit\phpunit\src\Framework\TestSuite.php(670): PHPUnit\Framework\TestSuite->run(Object(PHPUnit\Framework\TestResult))
#8 C:\Users\cimba\Documents\project\vendor\phpunit\phpunit\src\TextUI\TestRunner.php(673): PHPUnit\Framework\TestSuite->run(Object(PHPUnit\Framework\TestResult))
#9 C:\Users\cimba\Documents\project\vendor\phpunit\phpunit\src\TextUI\Command.php(143): PHPUnit\TextUI\TestRunner->run(Object(PHPUnit\Framework\TestSuite), Array, Array, true)
#10 C:\Users\cimba\Documents\project\vendor\phpunit\phpunit\src\TextUI\Command.php(96): PHPUnit\TextUI\Command->run(Array, true)
#11 C:\Users\cimba\Documents\project\bin\phpunit(11): PHPUnit\TextUI\Command::main()
#12 C:\Users\cimba\AppData\Local\Temp\ide-phpunit.php(224): require_once('C:\\Users\\cimba\\...')
#13 C:\Users\cimba\AppData\Local\Temp\ide-phpunit.php(173): IDE_PHPUnit_Loader::loadByAutoloader('C:\\Users\\cimba\\...')
#14 C:\Users\cimba\AppData\Local\Temp\ide-phpunit.php(228): IDE_PHPUnit_Loader::init()
#15 {main}

framework.yaml

framework:
    secret: '%env(APP_SECRET)%'
    http_method_override: false
    session:
        handler_id: null
        cookie_secure: auto
        cookie_samesite: lax
        storage_factory_id: session.storage.factory.native
    php_errors:
        log: true

when@test:
    framework:
        test: true
        session:
            storage_factory_id: session.storage.factory.mock_file