Perhaps someone will find this useful ;)

<?
           function rotN($s, $n){
                $s2 = "";
                for($i = 0; $i < strlen($s); $i++){
                    $char2 = $char = ord($s{$i});
                    $cap = $char & 32;

                    $char &= ~ $cap;
                    $char = $char > 64 && $char < 123 ? (($char - 65 + $n) % 26 + 65) : $char;
                    $char |= $cap;
                    if($char < 65 && $char2 > 64 || ($char > 90 && $char < 97 && ($char2 < 91 || $char2 > 96))) $char += 26;
                    else if($char > 122 && $char2 < 123) $char -= 52;
                    if(strtoupper(chr($char2)) === chr($char2)) $char = strtoupper(chr($char)); else $char = strtolower(chr($char));
                    $s2 .= $char;
                }
                return $s2;
            }
?>
It takes any string, $s, and any ROT value, $n. Just like str_rot13, it's both an encoder and decoder. To decode an encoded string, just pass -$n instead of $n.