Add tip for headers already send error.

[rehoehle]

Change for the problem PHPUnit headers already send.

symfony/symfony#25615

[javiereguiluz]

@rehoehle it's true that you need this to avoid the problem you mentioned ... but this is exactly the default value provided by Symfony:

• In old Symfony versions, the config_test.yml file included this config -> https://github.com/symfony/symfony-standard/blob/9884ed0ec1cc0631f55862e8343bc153dd9ac351/app/config/config_test.yml#L7
• In modern Symfony versions, the framework-bundle recipe includes this config -> https://github.com/symfony/recipes/blob/master/symfony/framework-bundle/3.3/config/packages/test/framework.yaml

So, should we still add this to the docs? What do you think @xabbuh? Thanks!

[rehoehle]

@javiereguiluz i had an error regards this:

symfony/symfony#25615

this is why @sroze mentioned to add that to the documentation. In my configuration that was created after upgrading to Symfony 3.4 the line was commented.

[xabbuh]

@rehoehle Could it be that you started your project at a time where this option was not set in the test environment?

[xabbuh]

for the record: symfony/recipes#333 is not so old

[javiereguiluz]

@rehoehle as you said in the original issue (symfony/symfony#25615) this has been worked like a charm for years ... and it will keep working. The problem was temporary and because of a bug in the recipe used by Symfony Flex when installing the FrameworkBundle. But this has been fixed so everything should be working again and users shouldn't need to make any change.

That's why I'm closing ... but we'll reopen if the problem wasn't really fixed. Thank you!