My own generic rotN function
<?php
function rotN($string, $n = 13)
{
$alphabet = join(range('a', 'z'));
$alen = strlen($alphabet);
$n %= $alen;
if($n < 0) {
$n += $alen;
}
if($n == 0) {
return $string;
}
$cycled = substr($alphabet . $alphabet, $n, $alen);
$alphabet = $alphabet . strtoupper($alphabet);
$cycled = $cycled . strtoupper($cycled);
return strtr($string, $alphabet, $cycled);
}
?>