Phpunit：在Functional测试中对类中的方法进行存根

I am performing functional tests with PHPUnit in a Symfony 4.3 project. All my test classes extend from TestCase.

I am having problems to stub the result of a method that makes a call to an external service. I don't want to check that this service works in my project functional tests, so I do the following:

public function testPutEndpoint()
{
    $stub = $this->createMock(ExternalRepository::class);
    $stub->method('put')->willReturn(['data'=> ['data']]);

    {
        list($responseCode, $content) = $this->makeCurl(
           //Here a curl to the endpoint of my project is done
        );

        $this->assertEquals(200, $responseCode);
    }

Here I see how the code continues going throw the real method ignoring the sub.

So my question is, how can I stub a method that is inside the logic to test, but I cannot call it directly in the test class?

Also, the constructor of the endpoint receives the Repository as an injection:

protected $externalRepository;

public function __construct(ExternalRepository $externalRepository)
{
    $this->externalRepository = $externalRepository;
    $this->commandBus = $commandBus;
}

At the moment the best solution I found to functional test the controller is:

1. Mock the external repository class, and stub the put method.
2. Create a Controller object and inject the stub
3. Invoke the Controller with the test request

4. Assert that the return of the controller matches what is expected

   public function testPut()
   {
       $stub = $this->createMock(CatalogRepository::class);
       $stub->method('put')->willReturn([0 => 200, 1 => ['data' => 12355]]);
   
       $request = Request::create('/path', Request::METHOD_PUT, [], [], [], [], ['body']);
       $putController = new PutController($stub);
   
       $response = $putController->__invoke($request);
       $expectedResponse = 'expected response'
   
       $this->assertEquals($expectedResponse, $response);
   }