Cannot send session cookie - headers already sent PHPUnit / Laravel

(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.)

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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();
  ...
}

One way to handle this in PHPUnit is to send output to stderr instead of stdout as is demonstrated by this answer.

phpunit --stderr

Or by adding stderr="true" in your phpunit.xml as is pointed out in this comment.

 /**
 * @test
 * @runInSeparateProcess
 */
public function testName(){
    //... your test
}