Deciphering

<?php

$cipher = readline();
$substring = explode(' ', readline());

for ($i = 0; $i < strlen($cipher); $i++) {
    $letter = $cipher[$i];
    if (ord($letter) >= ord('d') && ord($letter) <= ord('z') ||
        $letter === '{' || $letter === '}' || $letter === '|' || $letter === '#') {
        $cipher[$i] = chr(ord($letter) - 3);

    } else {
        echo 'This is not the book you are looking for.';
        return;
    }
}

$decipher = str_replace($substring[0], $substring[1], $cipher);

echo $decipher;