php check if date is y-m-d or d-m-y

function date_format($date){
  $date_parts = explode('-', $date);
  if(count($date_parts)==3){
    if(strlen($date_parts[0]) == 4){
      $formatted_date = $date;
    }elseif(strlen($date_parts[2]) == 4){
      $formatted_date = $date_parts[2] . '-' . $date_parts[1] . '-' . $date_parts[0];
    }else{
      $formatted_date = false;
    }
  }else{
    $formatted_date = false;
  }
  return $formatted_date;
}

date_format("09-12-2018");