Checkboxes::getCheckedCheckboxes

public static Checkboxes::getCheckedCheckboxes(array $input)

public static function getCheckedCheckboxes(array $input) {
  // Browsers do not include unchecked options in a form submission. The
  // FormAPI tries to normalize this to keep checkboxes consistent with other
  // form elements. Checkboxes show up as an array in the form of option_id =>
  // option_id|0, where integer 0 is an unchecked option.
  //
  // @see \Drupal\Core\Render\Element\Checkboxes::valueCallback()
  // @see https://www.w3.org/TR/html401/interact/forms.html#checkbox
  $checked = array_filter($input, function($value) {
    return $value !== 0;
  });
  return array_keys($checked);
}