List objects in a specific folder on Amazon S3

The answer is above however i figured i would supply a complete working example that can be copied and pasted directly into a php file and ran

use Aws\S3\S3Client;

require_once('PATH_TO_API/aws-autoloader.php');

$s3 = S3Client::factory(array(
    'key'    => 'YOUR_KEY',
    'secret' => 'YOUR_SECRET',
    'region' => 'us-west-2'
));

$bucket = 'YOUR_BUCKET_NAME';

$objects = $s3->getIterator('ListObjects', array(
    "Bucket" => $bucket,
    "Prefix" => 'some_folder/' //must have the trailing forward slash "/"
));

foreach ($objects as $object) {
    echo $object['Key'] . "<br>";
}

You need to use Prefix to restrict the search to a specific directory (a common prefix).

$objects = $client->getIterator('ListObjects', array(
    "Bucket" => $bucket,
    "Prefix" => "your-folder/"
)); 

"S3Client::factory is deprecated in SDK 3.x, otherwise the solution is valid" said by RADU

Here is the updated solution to help others who come across this answer:

# composer dependencies
require '/vendor/aws-autoloader.php';
//AWS access info  DEFINE command makes your Key and Secret more secure
if (!defined('awsAccessKey')) define('awsAccessKey', 'ACCESS_KEY_HERE');///  <- put in your key instead of ACCESS_KEY_HERE
if (!defined('awsSecretKey')) define('awsSecretKey', 'SECRET_KEY_HERE');///  <- put in your secret instead of SECRET_KEY_HERE


use Aws\S3\S3Client;

$config = [
    's3-access' => [
        'key' => awsAccessKey,
        'secret' => awsSecretKey,
        'bucket' => 'bucket',
        'region' => 'us-east-1', // 'US East (N. Virginia)' is 'us-east-1', research this because if you use the wrong one it won't work!
        'version' => 'latest',
        'acl' => 'public-read',
        'private-acl' => 'private'
    ]
];

# initializing s3
$s3 = Aws\S3\S3Client::factory([
    'credentials' => [
        'key' => $config['s3-access']['key'],
        'secret' => $config['s3-access']['secret']
    ],
    'version' => $config['s3-access']['version'],
    'region' => $config['s3-access']['region']
]);
$bucket = 'bucket';

$objects = $s3->getIterator('ListObjects', array(
    "Bucket" => $bucket,
    "Prefix" => 'filename' //must have the trailing forward slash for folders "folder/" or just type the beginning of a filename "pict" to list all of them like pict1, pict2, etc.
));

foreach ($objects as $object) {
    echo $object['Key'] . "<br>";
}