README

Edit composer.json.

{
    "require": {
        "symfony/filesystem": "~2.0"
    },
    "require-dev": {
        "phpunit/phpunit": "3.7.*",
        "ngyuki/composer-shared-installer": "*"
    },
    "extra": {
        "shared": {
            "exclude": [
                "phpunit/phpunit"
            ]
        }
    }
}

Run composer update command.

$ composer update

Create sample.php.

<?php
require 'vendor/autoload.php';

use Symfony\Component\Filesystem\Filesystem;

$ref = new ReflectionClass('Symfony\Component\Filesystem\Filesystem');
echo $ref->getFileName(), PHP_EOL;

$ref = new ReflectionClass('PHPUnit_Framework_TestCase');
echo $ref->getFileName(), PHP_EOL;

Run sample.php.

$ php sample.php

Symfony Filesystem will be installed global.

/home/your/.composer/shared/symfony/filesystem-2.3.6.0/Symfony/Component/Filesystem/Filesystem.php

PHPUnit will be installed local, because your exclude it.

/home/your/project/vendor/phpunit/phpunit/PHPUnit/Framework/TestCase.php