Unit testing headers with PHPUnit

headers.php

<?php
/**
 * Class that tries to send a header.
 */
class SendsHeaders {
    /**
     * Send a header to the user's browser.
     */
    public function sendHeader() {
        header('This is a header', true, 400);
    }
}

/**
 * Unit tests for SendHeaders class.
 */
class SendsHeadersTest extends PHPUnit_Framework_TestCase {
    /**
     * Array of headers that have been sent.
     * @var array
     */
    public static $headers;
    
    /**
     * Subject under test.
     * @var SendsHeaders
     */
    protected $sendsHeaders;

    /**
     * Use runkit to create a new header function.
     */
    public static function setUpBeforeClass() {
        if (!extension_loaded('runkit')) {
            return;
        }
        
        // First backup the real header function so we can restore it.
        runkit_function_rename('header', 'header_old');
        
        // Now, create a new header function that makes things testable.
        runkit_function_add(
            'header',
            '$string,$replace=true,$http_response_code=null',
            'SendsHeadersTest::$headers[] = array($string,$replace,$http_response_code);'
        );
    }

    /**
     * After we're done testing, restore the header function.
     */
    public static function tearDownAfterClass() {
        if (!extension_loaded('runkit')) {
            return;
        }

        // Get rid of our new header function.
        runkit_function_remove('header');
        
        // Move our backup to restore header to its original glory.
        runkit_function_rename('header_old', 'header');
    }
    
    /**
     * Set up our subject under test and global header state.
     */
    protected function setUp() {
        $this->sendsHeaders = new SendsHeaders();
        self::$headers = array();
    }
    
    /**
     * Unit test sending a header.
     * @requires extension runkit
     * @test
     */
    public function testSendHeader() {
        $expectedHeaders = array(
            array('This is a header', true, 400),
        );
        $this->sendsHeaders->sendHeader();
        $this->assertEquals($expectedHeaders, self::$headers);
    }
}