在对象方法中找不到类“S3Client”

My application on AWS EC2 stores image-files on AWS S3. I am using the following AWS code snippet to create a Guzzle client to the S3 API:

use Aws\S3\S3Client; // Namespace alias.
...
$s3_gClient = S3Client::factory(array(
   ...
   ));

This works fine - but is repeated at several places in the application. Now, I'd like to use a method to be called from all these places instead. I tried this:

  $s3ClientNamespaceString = "S3Client";
  $sg->prepare_aws_s3_guzzle_client($s3ClientNamespaceString);

with the method defined as:

  public function prepare_aws_s3_guzzle_client($s3ClientNamespaceString) {
    $s3_gClient = $s3ClientNamespaceString::factory(array(
       ...
       ));
    }

This is not working - I get a PHP error:

Fatal error: Class 'S3Client' not found

Is there any way to achieve what I am trying - i.e. to use a namespace alias within a method ?