ROT13 implementation

rot13.php

<?php

    /**
     * ROT13 implementation
     *
     * @see    https://en.wikipedia.org/wiki/ROT13
     * @param  string $message
     * @return string
     */
    function rot13($message)
    {
        $src_bytes = unpack('C*', $message);
        $dst_bytes = [];

        foreach($src_bytes as $byte)
        {
            if(($byte >= ord('A') && $byte <= ord('N')) || ($byte >= ord('a') && $byte <= ord('n')))
                $dst_bytes[] = $byte + 13;

            elseif(($byte >= ord('M') && $byte <= ord('Z')) || ($byte >= ord('m') && $byte <= ord('z')))
                $dst_bytes[] = $byte - 13;

            else
                $dst_bytes[] = $byte;
        }

        return call_user_func_array("pack", array_merge(array("C*"), $dst_bytes));
    }

    $message = 'Hello World';
    $encode  = rot13($message);
    $decode  = rot13($encode);

    print "{$encode}\n{$decode}\n";