Here's the scenario: you have code that will emit headers and content, for instance, a front controller. How do you test this?

The answer is remarkably simple, but non-obvious: namespaces.

Prerequisites

For this approach to work, the assumptions are:

• Your code emitting headers and output lives in a namespace other than the global namespace.

That's it. Considering that most PHP code you grab anymore does this, and most coding standards you run across will require this, it's a safe bet that you're already ready. If you're not, go refactor your code now, before continuing; you'll thank me later.

The technique

PHP introduced namespaces in PHP 5.3. Namespaces cover classes, as most of us are well aware, but they also cover constants and functions — a fact often overlooked, as before 5.6 (releasing next week!), you cannot import them via use statements!

That does not mean they cannot be defined and used, however — it just means that you need to manually import them, typically via a require or require_once statement. These are usually anathema in libraries, but for testing, they work just fine.

Here's an approach I took recently. I created a file that lives — this is the important bit, so pay attention — in the same namespace as the code emitting headers and output. This file defines several functions that live in the global (aka PHP's built-in) namespace, and an accumulator static object I can then use in my tests for assertions. Here's what it looks like:

namespace Some\Project;

abstract class Output
{
    public static $headers = array();
    public static $body;

    public static function reset()
    {
        self::$headers = array();
        self::$body = null;
    }
}

function headers_sent()
{
    return false;
}

function header($value)
{
    Output::$headers[] = $value;
}

function printf($text)
{
    Output::$body .= $text;
}

A few notes:

• headers_sent() always returns false here, as most emitters test for a boolean true value and bail early when that occurs.
• I used printf() here, as echo cannot be overridden due to being a PHP language construct and not an actual function. As such, if you use this technique, you will have to likely alter your emitter to call printf() instead of echo. The benefits, however, are worth it.
• I marked Output abstract, to prevent instantiation; it should only be used statically.

I place the above file within my test suite, usually under a TestAsset directory adjacent to the test itself; since it contains functions, I'll name the file Functions.php as well. This combination typically will prevent it from being autoloaded in any way, as the test directory will often not have autoloading defined, or will be under a separate namespace.

Inside your PHPUnit test suite, then, you would do the following:

namespace SomeTest\Project;

use PHPUnit_Framework_TestCase as TestCase;
use Some\Project\FrontController;
use Some\Project\Output;                 // <-- our Output class from above
require_once __DIR__ . '/TestAsset/Functions.php'; // <-- get our functions

class FrontControllerTest extends TestCase
{
    public function setUp()
    {
        Output::reset();
        /* ... */
    }

    public function tearDown()
    {
        Output::reset();
        /* ... */
    }
}

From here, you test as normal — but when you invoke methods that will cause headers or content to emit, you can now test to see what those contain:

    public function testEmitsExpectedHeadersAndContent()
    {
        /* ... */

        $this->assertContains('Content-Type: application/json', Output::$headers);
        $json = Output::$body;
        $data = json_decode($json, true);
        $this->assertArrayHasKey('foo', $data);
        $this->assertEquals('bar', $data['foo']);
    }

How it works

Why does this work?

PHP performs some magic when it resolves functions. With classes, it looks for a matching class either in the current namespace, or one that was imported (and potentially aliased); if a match is not found, it stops, and raises an error. With functions, however, it looks first in the current namespace, and if it isn't found, then looks in the global namespace. This last part is key — it means that if you redefine a function in the current namespace, it will be used in lieu of the original function defined by PHP. This also means that any code operating in the same namespace as the function — even if defined in another file — will use that function.

This technique just leverages this fact.