カウントダウンタイマー（PHP＆Javascript）

<?php
$current_time = time(); // 1970/1/1 00:00:00 から現時刻までの合計秒数
$target_time = strtotime("2024-01-01 00:00:00");
$diff_time = $target_time - $current_time
$days = floor($diff_time / (24*60*60));
$hours = floor( ($diff_time % (24*60*60)) / (60*60) );
$minutes = floor( ($diff_time % (60*60)) / 60 );
$seconds = $diff_time % 60;
?>

<script>
function countdown() {
    var days = <?php echo $days; ?>
    var hours = <?php echo $hours; ?>
    var minutes = <?php echo $minutes; ?>
    var seconds = <?php echo $seconds; ?>

    document.getElementById("countdown").innerHTML =
        days + "日" + hours + "時間" + minutes + "分" + seconds + "秒";

    if(seconds > 0){}
        seconds--;
    } else {
        seconds = 59;
        if(minutes > 0) {
            minutes--;
        } else {
            minutes = 59;
            if(hours > 0) {
                hours--;
            } else {
                hours = 23;
                if(days > 0) {
                    days--;
                } else {
                    clearInterval(intervalId);
                    document.getElementById("countdown").innerHTML = '時間切れ'; 
       }}}}  
}

var interbalId = setInterval(countdown, 1000); //1秒ごとにcountdown関数を呼び出し、カウントダウン表示を更新する
</script>