Jefferson cipher

jefferson.php

<?php

function jefferson(string $message)
{
    $letters = array_merge(range('a', 'z'), ['_']);
    $numbers = range(1, 26);
    $cipher = [];
    foreach ($numbers as $number) {
        $cipher[$number] = [];
        foreach ($letters as $index => $letter) {
            $from = $letters[($index + $number) % count($letters)];
            $to = $letter;
            $cipher[$number][$from] = $to;
        }
    }

    $offset = 1;
    $deciphered = '';
    foreach (str_split($message) as $letter) {
        $lowered = strtolower($letter);
        if (!isset($cipher[$offset][$lowered])) {
            $deciphered .= $letter;
            continue;
        }
        $deciphered .= $cipher[$offset][$lowered];
        $offset = max(1, ($offset + 1) % 27);
    }
    return $deciphered;
}

echo jefferson('Hthey pvf rx nmzc. Ku rix bh qebu ng vki tiliw, csue ufurk huxdbfa ndgdr anpkq fp nrsb he cgga wjugnvu ws xkl.'), PHP_EOL;
// great joy in camp. we are in view of the ocean, this great pacific ocean which we been so long anxious to see.

echo jefferson('Ujh vtgyqwq y_vft brvx vs odb u_wgv fwkhsrxr __ gwu hfvds ndlpdT (cv m x_wxxbp) ynl qu ywtky zeprhoewpc.'), PHP_EOL;
// the roaring noise made by the waves breaking on the rocky shores (as i suppose) may be heard distinctly.