A basic implementation of a Caesar cipher

caesar_cipher.php

<?php
function rollover($max, $value) {
	return $value < $max ? $value : rollover($max, $value - $max);
}

function caesar_cipher($string, $offset = 3) {
	$letter_numbers = array_map('ord', str_split(strtolower($string)));
	foreach($letter_numbers as &$number)
		if($number >= 97 && $number <= 122)
			$number = rollover(25, $number - 97 + $offset) + 97;
	return implode('', array_map('chr', $letter_numbers));
}

print caesar_cipher('hejsan hoppsan - jao!');