Element::children

public static Element::children(array &$elements, $sort = FALSE)

public static function children(array &$elements, $sort = FALSE) {
  // Do not attempt to sort elements which have already been sorted.
  $sort = isset($elements['#sorted']) ? !$elements['#sorted'] : $sort;

  // Filter out properties from the element, leaving only children.
  $count = count($elements);
  $child_weights = array();
  $i = 0;
  $sortable = FALSE;
  foreach ($elements as $key => $value) {
    if ($key === '' || $key[0] !== '#') {
      if (is_array($value)) {
        if (isset($value['#weight'])) {
          $weight = $value['#weight'];
          $sortable = TRUE;
        }
        else {
          $weight = 0;
        }
        // Supports weight with up to three digit precision and conserve
        // the insertion order.
        $child_weights[$key] = floor($weight * 1000) + $i / $count;
      }
      // Only trigger an error if the value is not null.
      // @see https://www.drupal.org/node/1283892
      elseif (isset($value)) {
        trigger_error(SafeMarkup::format('"@key" is an invalid render array key', array('@key' => $key)), E_USER_ERROR);
      }
    }
    $i++;
  }

  // Sort the children if necessary.
  if ($sort && $sortable) {
    asort($child_weights);
    // Put the sorted children back into $elements in the correct order, to
    // preserve sorting if the same element is passed through
    // \Drupal\Core\Render\Element::children() twice.
    foreach ($child_weights as $key => $weight) {
      $value = $elements[$key];
      unset($elements[$key]);
      $elements[$key] = $value;
    }
    $elements['#sorted'] = TRUE;
  }

  return array_keys($child_weights);
}