require 'vendor/autoload.php';
use Aws\S3\S3Client;
use Aws\Exception\AwsException;

$config =
[
    'version' => 'latest',
    'region' => '',
    'endpoint' => '[cluster-url]',
    'credentials' =>
    [
        'key' => '[access-key]',
        'secret' => '[secret-key]',
    ],
];

$client = new \Aws\S3\S3Client($config);

$result = $client->listObjects([
    'Bucket' => 'bucket-name',
    'Prefix' => 'sub/directory/'
]);

foreach ($result['Contents'] as $key => $object) {
    // Is the value of prefix.
    if (0 === $key) {
        continue;
    }

    echo $object['Key'] . "<br>";
}