无法发送会话cookie - 标头已经发送过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.)

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.

In Laravel I avoided the issue by checking the environment and avoiding problematic code for 'testing' environment.

// Code working in Laravel 4.2
if(App::environment() != 'testing') {
        // this will be skipped when testing
        setcookie('key', $val, time() + (86400 * 999), '/');
}

NOTE - Warning: this means you cannot test this piece of code!