/**
  * Returns a signed Amazon S3 download URL.
  *
  * @param $bucket       string  Bucket name
  * @param $file_name    string  File name (URI)
  * @return string       The signed download URL
  */
 public static function get_s3_url($bucket, $file_name)
 {
     $s3_url = '';
     $options = get_option('wp-license-manager-settings');
     if (Wp_License_Manager_S3::use_official_api()) {
         write_log('Using official API ');
         $s3_url = Wp_License_Manager_S3_Official::get_s3_url($options['aws_key'], $options['aws_secret'], $bucket, $file_name);
     } else {
         write_log('Using fallback API ');
         $s3_url = Wp_License_Manager_S3_Fallback::get_s3_url($options['aws_key'], $options['aws_secret'], $bucket, $file_name);
     }
     return $s3_url;
 }