Website Review and Suggestions

Function ROT13($rot13text) {

    $rot13text_rotated = "";

    for ($i = 0; $i <= strlen($rot13text); $i++) {

      $k = ord(substr($rot13text, $i, 1));

      if ($k >= 97 and $k <= 109) {
        $k = $k + 13;
      } elseif ($k >= 110 and $k <= 122) {
        $k = $k - 13;
      } elseif ($k >= 65 and $k <= 77) {
        $k = $k + 13;
      } elseif ($k >= 78 and $k <= 90) {
        $k = $k - 13;
      }

        $rot13text_rotated = $rot13text_rotated . Chr($k);
    }

     return $rot13text_rotated;

}