Composer autoloader not loading GuzzleHttp\ClientInterface

Question

I'm trying to use Guzzle, but I'm getting the following fatal error:

Fatal error: Class 'GuzzleHttp\ClientInterface' not found in /var/www/myapp/vendor/guzzlehttp/guzzle/src/functions.php on line 13

I'm autoloading with composer autoloader:

require 'vendor/autoload.php';

use Guzzle\Http\Client;

$client = new Client();

$requests = Array(
    $client->createRequest('GET', 'ams1.myapp.com:8080/api/ffmpeg_make_snapshots.php'),
    $client->createRequest('GET', 'ams2.myapp.com:8080/api/ffmpeg_make_snapshots.php'),
    $client->createRequest('GET', 'ams3.myapp.com:8080/api/ffmpeg_make_snapshots.php')
);

$client->sendAll($requests);

What is happening?

EDIT

My composer.json:

{
    "require": {
        "guzzlehttp/guzzle": "~4"
    }
}

EDIT 2

My composer.lock

{
    "_readme": [
        "This file locks the dependencies of your project to a known state",
        "Read more about it at http://getcomposer.org/doc/01-basic-usage.md#composer-lock-the-lock-file"
    ],
    "hash": "33c003b6e3e2dc0144146f9b652a6719",
    "packages": [
        {
            "name": "guzzlehttp/guzzle",
            "version": "4.0.0",
            "source": {
                "type": "git",
                "url": "https://github.com/guzzle/guzzle.git",
                "reference": "4063f08ca434efac12bf7a3db0d370b1c451345b"
            },
            "dist": {
                "type": "zip",
                "url": "https://api.github.com/repos/guzzle/guzzle/zipball/4063f08ca434efac12bf7a3db0d370b1c451345b",
                "reference": "4063f08ca434efac12bf7a3db0d370b1c451345b",
                "shasum": ""
            },
            "require": {
                "guzzlehttp/streams": "1.*",
                "php": ">=5.4.0"
            },
            "require-dev": {
                "ext-curl": "*",
                "phpunit/phpunit": "4.*",
                "psr/log": "~1"
            },
            "type": "library",
            "extra": {
                "branch-alias": {
                    "dev-master": "4.0.x-dev"
                }
            },
            "autoload": {
                "psr-4": {
                    "GuzzleHttp\\": "src/"
                },
                "files": [
                    "src/functions.php"
                ]
            },
            "notification-url": "https://packagist.org/downloads/",
            "license": [
                "MIT"
            ],
            "authors": [
                {
                    "name": "Michael Dowling",
                    "email": "[email protected]",
                    "homepage": "https://github.com/mtdowling"
                }
            ],
            "description": "Guzzle is a PHP HTTP client library and framework for building RESTful web service clients",
            "homepage": "http://guzzlephp.org/",
            "keywords": [
                "client",
                "curl",
                "framework",
                "http",
                "http client",
                "rest",
                "web service"
            ],
            "time": "2014-03-29 23:11:36"
        },
        {
            "name": "guzzlehttp/streams",
            "version": "1.4.0",
            "source": {
                "type": "git",
                "url": "https://github.com/guzzle/streams.git",
                "reference": "3b761a328e5ed6ed519e960aded95d7acbe77894"
            },
            "dist": {
                "type": "zip",
                "url": "https://api.github.com/repos/guzzle/streams/zipball/3b761a328e5ed6ed519e960aded95d7acbe77894",
                "reference": "3b761a328e5ed6ed519e960aded95d7acbe77894",
                "shasum": ""
            },
            "require": {
                "php": ">=5.4.0"
            },
            "require-dev": {
                "phpunit/phpunit": "~4.0"
            },
            "type": "library",
            "extra": {
                "branch-alias": {
                    "dev-master": "1.4.x-dev"
                }
            },
            "autoload": {
                "psr-4": {
                    "GuzzleHttp\\Stream\\": "src/"
                },
                "files": [
                    "src/functions.php"
                ]
            },
            "notification-url": "https://packagist.org/downloads/",
            "license": [
                "MIT"
            ],
            "authors": [
                {
                    "name": "Michael Dowling",
                    "email": "[email protected]",
                    "homepage": "https://github.com/mtdowling"
                }
            ],
            "description": "Provides a simple abstraction over streams of data (Guzzle 4+)",
            "homepage": "http://guzzlephp.org/",
            "keywords": [
                "Guzzle",
                "stream"
            ],
            "time": "2014-07-19 18:43:42"
        }
    ],
    "packages-dev": [],
    "aliases": [],
    "minimum-stability": "stable",
    "stability-flags": [],
    "platform": [],
    "platform-dev": []
}

EDIT 3

composer config -l output:

[repositories.packagist.type] composer
[repositories.packagist.url] https?://packagist.org
[repositories.packagist.allow_ssl_downgrade] true
[process-timeout] 300
[use-include-path] false
[preferred-install] auto
[notify-on-install] true
[github-protocols] [git, https]
[vendor-dir] vendor
[bin-dir] {$vendor-dir}/bin (vendor/bin)
[cache-dir] C:/Users/User/AppData/Local/Composer
[cache-files-dir] {$cache-dir}/files (C:/Users/User/AppData/Local/Composer/files)
[cache-repo-dir] {$cache-dir}/repo (C:/Users/User/AppData/Local/Composer/repo)
[cache-vcs-dir] {$cache-dir}/vcs (C:/Users/User/AppData/Local/Composer/vcs)
[cache-ttl] 15552000
[cache-files-ttl] 15552000
[cache-files-maxsize] 300MiB (314572800)
[discard-changes] false
[prepend-autoloader] true
[github-domains] [github.com]
[home] C:/Users/User/AppData/Roaming/Compose

Answer 1

The namespace for guzzle 4 is GuzzleHttp in guzzle 3 the namespace was simply Guzzle.

A composer.json of:

{
    "require": {
        "guzzlehttp/guzzle": "~4"
    }
}

Should allow you to run a php script of:

require 'vendor/autoload.php';

use GuzzleHttp\Client;

$client = new Client();

$requests = Array(
    $client->createRequest('GET', 'ams1.myapp.com:8080/api/ffmpeg_make_snapshots.php'),
    $client->createRequest('GET', 'ams2.myapp.com:8080/api/ffmpeg_make_snapshots.php'),
    $client->createRequest('GET', 'ams3.myapp.com:8080/api/ffmpeg_make_snapshots.php'),
);

$client->sendAll($requests);

If the autoloader still fails after changing the namespace it could be that your version of composer is out of date and does not recognize PSR4 autoloading. If there is no file in vendor/composer/autoload_psr4.php try doing a composer self-update followed by a composer dump-autoload to see if the problem is resolved.