How do you use PHPUnit to test a function if that function is supposed to kill PHP?

As every tests are run by the same PHPUnit process, if you use exit/die in your PHP code, you will kill everything -- as you noticed ^^

So, you have to find another solution, yes -- like returning instead of dying ; or throwing an exception (you can test if some tested code has thrown an expected exception).

Maybe PHPUnit 3.4 and it's --process-isolation switch (see Optionally execute each test using a separate PHP process) might help (by not having everything dying), but you still wouldn't be able to get the result of the test, if PHPUnit doesn't get the control back.

I've had this problem a couple of times ; solved it by returning instead of dying -- even returning several times, if needed, to go back "high enough" in the call stack ^^
In the end, I suppose I don't have any "die" anymore in my application... It's probably better, when thinking about MVC, btw.

It's obviously an old question but my suggestion would be to move the code that die()'s into a separate method that you can then mock.

As an example, instead of having this:

class SomeClass
{
    public function do()
    {
        exit(1);
        // or
        die('Message');
    }
}

do this:

class SomeClass
{
    public function do()
    {
        $this->terminate(123);
        // or
        $this->terminate('Message');
    }

    protected function terminate($code = 0)
    {
        exit($code);
    }

    // or 
    protected function terminate($message = '')
    {
        die($message);
    }
}

That way you can easily mock the terminate method and you don't have to worry about the script terminating without you being able to catch it.

Your test would look something like this:

class SomeClassTest extends \PHPUnit_Framework_TestCase
{

    /**
     * @expectedExceptionCode 123
     */
    public function testDoFail()
    {
        $mock = $this->getMock('SomeClass');
        $mock->expects($this->any())
             ->method('terminate')
             ->will($this->returnCallback(function($code) {
                 throw new \Exception($code);
             }));

        // run to fail
        $mock->do();
    }
}

I haven't tested the code but should be pretty close to a working state.

There's no need to change the code just to be able to test it, you can simply use set_exit_overload() (provided by test_helpers from same author as PHPUnit).