app.php

<?php

echo 'Foo!';

LegacyFunctionalTest.php

<?php

namespace Acme\Controller;

class FooControllerTest extends \PHPUnit_Framework_TestCase
{
    public function testHook()
    {
        $_SERVER['HTTP_HOST'] = 'localhost';
        $_SERVER['HTTP_USER_AGENT'] = 'Foobar Agent';
        $_SERVER['REQUEST_URI'] = '/app.php';
        $_SERVER['REQUEST_METHOD'] = 'GET';
        require_once __DIR__ . '/../../web/app.php';

        $this->expectOutputString('Foo!');
    }
}

phpunit.xml

<?xml version="1.0" encoding="UTF-8"?>

<!-- http://www.phpunit.de/manual/current/en/appendixes.configuration.html -->
<phpunit
        backupGlobals               = "false"
        backupStaticAttributes      = "false"
        colors                      = "true"
        convertErrorsToExceptions   = "true"
        convertNoticesToExceptions  = "true"
        convertWarningsToExceptions = "true"
        processIsolation            = "true"
        stopOnFailure               = "false"
        syntaxCheck                 = "false"
        bootstrap                   = "vendor/autoload.php" >

    <testsuites>
        <testsuite name="Acme Test Suite">
            <directory>tests</directory>
        </testsuite>
    </testsuites>

</phpunit>