Cannot send session cookie - headers already sent PHPUnit / Laravel

Question

I have this strange problem when i call the parent::setUp() on my TestCase class for unit test a class when i run phpunit it throw me this error:

Answer 1

(UPDATE: Thanks to comment by Louis Charette, this will only work in php 7.1 and earlier (and reading the bug report, it sounds like a regression they don't intend to fix. The better solution is therefore Jeff Puckett's (https://stackoverflow.com/a/38045422/841830), of giving the --stderr flag on the commandline to phpunit. This keeps stdout for your code, stderr for phpunit, and so they don't clash.)

---

The problem is that you have some code, perhaps deep in the framework you use, that calls session_start(). That, in turn, wants to send a cookie. But PHPUnit has already started writing output to stdout.

The point to understand here is that this is just a unit test, no-one cares about the header. So just suppress the error message. And the way you do that, without altering the system-under-test, is to call session_start() in your own unit test (either before parent::setUp() or inside that setUp function). And use the @ prefix to suppress errors. e.g.

function setUp() {
  @session_start();
  parent::setUp();
  ...
}