gistfile1.txt

<?php

class CifraDeCesar
{
    const IN = 0;
    const OUT = 1;

    private $interval;

    public function __construct($interval)
    {
        $this->setInterval($interval);
    }

    public function encode($term)
    {
        $term_iterable = str_split($term);
        $result = [];
        foreach ($term_iterable as $letter) {
            $result[] = $this->translate($letter, self::IN);
        }

        return implode('', $result);
    }

    public function decode($term)
    {
        $term_iterable = str_split($term);
        $result = [];
        foreach ($term_iterable as $letter) {
            $result[] = $this->translate($letter, self::OUT);
        }

        return implode('', $result);
    }

    private function setInterval($interval)
    {
        if ($interval < 1) {
            throw new \Exception("Interval must be bigger of 1");
        }
        $this->interval = $interval;
    }

    private function translate($letter, $mode)
    {
        $alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";

        $pos = strpos($alphabet, $letter);
        if ($pos === false) {
            return $letter;
        }

        if ($mode === 0) {
            $pos += $this->interval;
        }

        if ($mode === 1) {
            $pos -= $this->interval;
        }
        return $alphabet[$pos];
    }
}

$term = 'Erik Figueiredo';

$cifraDeCesar = new CifraDeCesar(3);
$encoded = $cifraDeCesar->encode($term);

echo $encoded;
echo PHP_EOL;

echo $cifraDeCesar->decode($encoded);