使用 strtotime() 从字符串（各种时间单位）中减去间隔；

我正在创建一个票务系统。我已经存储了票证创建日期和当前日期之间的间隔。

每张票证都有一个存储为字符串的解决时间限制 ($targetTime) .. 15 分钟、4 小时等。

我试图通过从时间限制字符串中减去间隔来计算剩余时间。

我尝试在 $targetTime 上使用 strtotime 但我很难理解它应该如何格式化。

$then = new DateTime($query->fetchColumn());
$now = new DateTime();
$interval=date_diff($then, $now);
$targetTime = strtotime($targetTime);

$timeRemaining = $targetTime - $interval;

$targetTime = 1649991600;
echo timeLeft($targetTime);

function timeLeft($timestamp) {

   $strTime = array("second", "minute", "hour", "day", "month", "year");
   $length = array("60","60","24","30","12","10");

   $currentTime = time();
   if($currentTime <= $timestamp) {
        $diff     = $timestamp-time();
        for($i = 0; $diff >= $length[$i] && $i < count($length)-1; $i++) {
        $diff = $diff / $length[$i];
        }


        $diff = round($diff);
        if ($diff == 1) {
            return $diff . " " . $strTime[$i] . " left ";
        }else{
            return $diff . " " . $strTime[$i] . "s left ";
        }
   }
}