PHP & Composer, how do I combine composer.json files

Question

Can someone explain how I'm supposed to use composer with a php. I have a composer.json file in my doc root that downloads the core packages for my project, but then when I want to add another project like google+ php sdk found here https://github.com/googleplus/gplus-quickstart-php/

what do I do with that composer.json file? do i combine them manually? Do I just download that composer.json into a different dir?

my current composer.json file looks like this

 {
"name": "fuel/fuel",
"type": "metapackage",
"description": "The FuelPHP framework",
"keywords": ["framework"],
"homepage": "http://fuelphp.com",
"license": "MIT",
"authors": [
    {
        "name": "FuelPHP Development Team",
        "email": "team@fuelphp.com"
    }
],
"support": {
    "irc": "irc://irc.freenode.org/fuelphp",
    "forum": "http://fuelphp.com/forums"
},
"require": {
    "php": ">=5.3.3",
    "monolog/monolog": "1.5.*",
    "fuelphp/upload": "2.0",
    "omissis/php-cloudfiles": "dev-master",
    "mustache/mustache": "*"
},
"suggest": {
    "mustache/mustache": "Allow Mustache templating with the Parser package",
    "smarty/smarty": "Allow Smarty templating with the Parser package",
    "twig/twig": "Allow Twig templating with the Parser package",
    "mthaml/mthaml": "Allow Haml templating with Twig supports with the Parser package"
},
"config": {
    "vendor-dir": "fuel/vendor"
},
"minimum-stability": "dev"
}

The g+ composer.json file looks like

 {
"name": "googleplus/quickstart",
"description": "This quick-start app is built in PHP and lets you get started with the Google+ platform in a few minutes.",
"license": "Apache-2.0",
"repositories": [
    {
        "type": "package",
        "package": {
            "name": "google/google-api-php-client",
            "version": "0.6.2",
            "dist": {
                "url": "http://google-api-php-client.googlecode.com/files/google-api-php-client-0.6.2.tar.gz",
                "type": "tar"
            },
            "autoload": {
                "classmap": [
                    "src/"
                ]
            }
        }
    }
],
"require": {
    "silex/silex": "1.0.*@dev",
    "twig/twig": ">=1.8,<2.0-dev",
    "google/google-api-php-client": "0.6.2"
}
}

Answer 1

Simply add this into the require block of your original composer file:

"googleplus/quickstart": "*",

Answer 2

I have little experience with Composer, but I can tell you that when you require a lib, you add that lib to your main composer.json file. Upon installation or update, the new lib will be downloaded and its composer.json file will be read by Composer; its dependencies will be automatically downloaded, and so on.

So we could say you're not supposed to download your requirements manually, you have to use Composer for that, it will take care of it for you.