I am writing functional tests with Symfony2.
I have a controller that calls a getImage() function which streams an image file as follows:
The return value of sendContent (rather than getContent) is the callback that you've set. getContent actually just returns false in Symfony2
Using sendContent you can enable the output buffer and assign the content to that for your tests, like so:
$client = static::createClient();
$client->request('GET', $url);
// Enable the output buffer
ob_start();
// Send the response to the output buffer
$client->getResponse()->sendContent();
// Get the contents of the output buffer
$content = ob_get_contents();
// Clean the output buffer and end it
ob_end_clean();
You can read more on the output buffer here
The API for StreamResponse is here