Subtracting an interval from string (various units of time) using strtotime();

Question

I am creating a ticket system. i have stored the interval between the tickets creation date & the current date.

Each ticket has a resolution time limit ($targetTime) stored as a string .. 15 minutes, 4 hours, etc.

I am trying to calculate the time remaining by subtracting the interval from the time limit string.

I have tried to use strtotime on $targetTime but I am struggling to understand how it should be formatted.

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

$timeRemaining = $targetTime - $interval;

Answer 1

$targetTime = 1649991600;
echo timeLeft($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 ";
        }
   }
}