Caesar Cipher

CaesarCipher.php

<?php
/**
 * Caesar Cipher
 */
$msg = 'GUVFVFZLFRPERGZRFFNTR';

$key = 13;

$mode = 'decrypt';

$letters = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ';

$trans = '';

$msg = strtoupper($msg);

for ($i = 0; $i < strlen($msg); $i++) {
    if (strpos($letters, $msg[$i]) !== false) {
        $num = strpos($letters, $msg[$i]);
        
        if ($mode == 'encrypt') {
            $num += $key;
        } else {
            $num -= $key;
        }
        
        $cnt = strlen($letters);
        if ($num >= $cnt) {
            $num -= $cnt;
        } else {
            $num += $cnt;
        }
        if ($num >= $cnt) {
            $num = $num - $cnt;
        }
        $trans .= $letters[$num];
    }    
}
echo $trans . PHP_EOL;