/**
   * Flattens and concatenates an array according to what QuickPay expects as parameters.
   *
   * @param array $parameters
   *   The array to be flattened.
   * @param string $prefix
   *   The prefix to add in front of every key.
   * @return array
   *   Returns a flattened array with concatenated keys.
   */
  public function formatParameterArray(array $parameters, $prefix = '') {
    $result = [];

    foreach ($parameters as $key => $value) {
      if (empty($prefix)) {
        $new_key = $key;
      } else {
        $new_key = $prefix.'['.$key.']';
      }

      if (is_array($value)) {
        $result = array_merge($result, $this->formatParameterArray($value, $new_key));
      } else {
        $result[$new_key] = $value;
      }
    }

    return $result;
  }