I've got a small web application up and running, with phpunit testing included. It's built with composer, and has a pretty standard directory structure:

src/
    application files
tests/
    bootstrap.php
    test files
web/
    index.php
.travis.yml
composer.json
composer.lock
phpunit.xml

phpunit.xml contents:

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

<phpunit boostrap="./tests/bootstrap.php">
    <testsuites>
        <testsuite name="Chronos">
            <directory>./tests</directory>
        </testsuite>
    </testsuites>
</phpunit>

bootstrap.php contents:

<?php

require __DIR__ . "/../vendor/autoload.php";

My Problem

Running the following command works perfectly and my tests pass:

$ vendor/bin/phpunit
PHPUnit 4.8.26 by Sebastian Bergmann and contributors.

..

Time: 104 ms, Memory: 4.00MB

OK (2 tests, 2 assertions)

However, it fails when I run my globally installed version of phpunit:

$ phpunit
PHPUnit 4.8.26 by Sebastian Bergmann and contributors.

EE

Time: 117 ms, Memory: 4.00MB

There were 2 errors:

1) WelcomeTest::testReturnsAPayload
Error: Class 'Equip\Payload' not found

/Users/tony/Documents/projects/chronos/tests/Domain/WelcomeTest.php:13

2) WelcomeTest::testReturnsOkStatus
Error: Class 'Equip\Payload' not found

/Users/tony/Documents/projects/chronos/tests/Domain/WelcomeTest.php:13

FAILURES!
Tests: 2, Assertions: 0, Errors: 2.

It would seem that running the global installation of phpunit in my project's root directory does not include/resolve my phpunit.xml file (which specifies the autoloader). I get the same results on TravisCI.

Does anyone know why this is happening? I could just specify the local version of phpunit in my .travis.yml file, but I'd rather know what causes the difference here.