Шифр цезаря

CesarCipher.php

<?php

class CesarCipher
{
    private $alphabet;
    private $shift;
    
    /**
     * @param string $alphabet
     */
    public function setAlphabet($alphabet)
    {
        $this->alphabet = $alphabet;
    }

    /**
     * @param int $shift
     *
     * The $shift parameter must be a positive integer
     */
    public function setShift($shift)
    {
        $this->shift = $shift;
    }

    /**
     * @param string $plainText
     * @return string
     */
    public function encode($plainText)
    {
        $encodedText = [];
        $plainTextLength = strlen($plainText);
        $alphabetLength = strlen($this->alphabet);
        for ($index = 0; $index < $plainTextLength; $index++) {
            $position = strpos($this->alphabet, $plainText[$index]);
            $encodedText[] = $position !== false ? $this->alphabet[($position + $this->shift)%$alphabetLength] : $plainText[$index];
        }
        return implode($encodedText, '');
    }
    
    /**
     * @param $encodedText
     * @return string
     */
    public function decode($encodedText) 
    {
        $decodedText = [];
        $plainTextLength = strlen($encodedText);
        $alphabetLength = strlen($this->alphabet);
        
        for ($index = 0; $index < $plainTextLength; $index++) {
            
            $position = strpos($this->alphabet, $encodedText[$index]);
            $n = ($position - $this->shift)%$alphabetLength;
            $element = $n > 0 ? $this->alphabet[$n] : $this->alphabet[abs(($n + $alphabetLength)%$alphabetLength)];
            $decodedText[] = $position !== false ? $element : $encodedText[$index];
        }
        return implode($decodedText, '');
    }
}

$n = new CesarCipher();
$n->setAlphabet("abcdefghijklmnopqrstuvwxyz");
$n->setShift(3);
$s = "The quick brown fox jumps over the lazy dog";
echo $s."\n";
$s = $n->encode($s);
echo $s."\n";
echo $n->decode($s);