why do i get "acces denied" when trying to use file_get_contents() in a file in a bucket s3?

i'm trying to use php functions for reading and writing files in a s3 bucket,i've tried uploading some files and it works fine , but when trying this code :

 ini_set('display_errors', 1);     
    require 'vendor/autoload.php';

    use Aws\S3\S3Client;
    use Aws\S3\Exception\S3Exception;

    // Instantiate an S3 client  
    $client = S3Client::factory(array(
            'key' => 'key',
            'secret' => 'secret-key',
            'region' => 'region'
        ));

    $client->registerStreamWrapper();
    $data = file_get_contents("http://s3bucket_name/file.txt");
    echo $data;

i get this error :

AccessDenied Access Denied

plz any idea. Thanks.