Caesar cipher encryption technique https://en.wikipedia.org/wiki/Caesar_cipher

Caesar cipher .php

<?php

function caesar_cipher_smart($shift, $input) {
	$a = str_split('ABCDEFGHIJKLMNOPQRSTUVWXYZ');
	$p = str_split($input);
	foreach ($p as &$v)
	if (($k = array_search($v, $a)) !== false) {
		$t = $k+$shift;
		if ($t < 0) $t = count($a)+$t;
		$v = $a[$t];
	}
	return implode('', $p);
}

function caesar_cipher_lazy($shift, $input) {
	$a = str_split('ABCDEFGHIJKLMNOPQRSTUVWXYZ');
	$b = str_split('XYZABCDEFGHIJKLMNOPQRSTUVW');
	$p = str_split($input);
	foreach ($p as &$v)
		if (($k = array_search($v, $a)) !== false)
			$v = $b[$k];
	return implode('', $p);
}

$input = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ';

echo 'Input "'.$input.'"';
echo '<br> -> Output (smart) "'.caesar_cipher_smart(-3, $input).'"';
echo '<br> -> Output (lazy)  "'.caesar_cipher_lazy(-3, $input).'"';