trying to use Composer

I am trying to load up the AWS PHP sdk. The install instructs say to use composer, which I did. It then states:

Require the Composer autoloader in your scripts.

Code:

<?php
  require '/path/to/vendor/autoload.php';
?>

what is /path/to/vendor/ mean? what vendor? Are they talking about the vendor directory in my CI project? I see an autoload.php in my vendor directory in my CI project. But how do I path to this? from the root of the project which would be /xxx/vendor/autoload.php.  I don't know what xxx would be? My prject is call sub_crud so would it be /sub_crud/vendor/autoload.php ?

I copied /aws/s3 into third_party/

<?php if ( ! defined('BASEPATH')) exit('No direct script access allowed');
 require 'vendor/autoload.php';

use third_party\Aws\S3\S3Client;

But looking in the S3 directory, I see all the files are .java. Does that mean I have the wrong version of the sdk? Should they be .php?

But when I start up the AWS\S3\S3Client can't be found. 

proof that an old dog can learn new tricks