code: public static function strCaseReverse($pValue)
      {
          $characters = self::mbStrSplit($pValue);
          foreach ($characters as &$character) {
              if (self::mbIsUpper($character)) {
                  $character = mb_strtolower($character, 'UTF-8');
              } else {
                  $character = mb_strtoupper($character, 'UTF-8');
              }
          }
  
          return implode('', $characters);
      }
docs: Reverse the case of a string, so that all uppercase characters become lowercase
  and all lowercase characters become uppercase.
  
  @param string $pValue UTF-8 encoded string
  
  @return string