PHP でタイムスタンプから相対日付/時刻文字列を生成するにはどうすればよいですか?

function time2str($ts)
{
    // Convert to timestamp if not already
    if (!ctype_digit($ts)) {
        $ts = strtotime($ts);
    }

    // Calculate difference between current time and timestamp
    $diff = time() - $ts;

    // Past Timestamps
    if ($diff > 0) {
        $day_diff = floor($diff / 86400);
        switch ($day_diff) {
            case 0:
                if ($diff < 60) {
                    return 'just now';
                } elseif ($diff < 120) {
                    return '1 minute ago';
                } elseif ($diff < 3600) {
                    return floor($diff / 60) . ' minutes ago';
                } elseif ($diff < 7200) {
                    return '1 hour ago';
                } elseif ($diff < 86400) {
                    return floor($diff / 3600) . ' hours ago';
                }
                break;
            case 1:
                return 'Yesterday';
            default:
                if ($day_diff < 7) {
                    return $day_diff . ' days ago';
                } elseif ($day_diff < 31) {
                    return ceil($day_diff / 7) . ' weeks ago';
                } elseif ($day_diff < 60) {
                    return 'last month';
                } else {
                    return date('F Y', $ts);
                }
        }
    }

    // Future Timestamps
    else {
        $diff = abs($diff);
        $day_diff = floor($diff / 86400);
        switch ($day_diff) {
            case 0:
                if ($diff < 120) {
                    return 'in a minute';
                } elseif ($diff < 3600) {
                    return 'in ' . floor($diff / 60) . ' minutes';
                } elseif ($diff < 7200) {
                    return 'in an hour';
                } elseif ($diff < 86400) {
                    return 'in ' . floor($diff / 3600) . ' hours';
                }
                break;
            case 1:
                return 'Tomorrow';
            default:
                if ($day_diff < 4) {
                    return date('l', $ts);
                } elseif ($day_diff < 7 + (7 - date('w'))) {
                    return 'next week';
                } elseif (ceil($day_diff / 7) < 4) {
                    return 'in ' . ceil($day_diff / 7) . ' weeks';
                } elseif (date('n', $ts) == date('n') + 1) {
                    return 'next month';
                } else {
                    return date('F Y', $ts);
                }
        }
    }
}

echo time2str('2023-09-20 10:00:00'); // Output: "in 27 days"
echo time2str('2022-09-20 10:00:00'); // Output: "last year"
echo time2str('2022-09-20T10:00:00+00:00'); // Output: "last year"