ArrayFlatterator::flattenWithCheckKeys

private function flattenWithCheckKeys(array $input): array
{
	foreach ($input as $key => $value)
	{
		if (is_array($value) && in_array($key, $this->processedKeys, true))
		{
			$newItems = $this->getFlattenFields($value, $key);
			foreach ($newItems as $newKey => $newValue)
			{
				$input[$newKey] = $newValue;
			}
		}
	}

	return $input;
}