How can I calculate time remaining?

[Chappie]

Hello.

I'm considering building an auction feature in to my site, however I'm unable to figure out how to get started.

At present, reading from a database, I have selected the start date and time, then selected the duration in days.

$start_time = openssl_decrypt($row_auction['start_time'], $ciphering, $decryption_key, $options, $decryption_iv);
$duration = $row_auction['listing_days'];
$ending = date('Y-m-d H:i:s', strtotime($start_time. ' + $duration days'));

And then, I'd like to work out days, hours, minutes an maybe seconds remaining or just time remaining.
I've tried dateDiff, but got a blank page.
May I ask if there is an easy way of achieving my goal?

At present I've written, which hopefuly calculated end date/Time:-

<?php
try {
    $conn = new PDO("mysql:host=$servername;dbname=$dbname", $username, $password);
    // set the PDO error mode to exception
    $conn->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
    $results = $conn->query('SELECT * FROM item_bid');
    while ($row = $results->fetch()) {
        $bidder_id = openssl_decrypt($row['bidder_id'], $ciphering, $decryption_key, $options, $decryption_iv);
        $auction_id = openssl_decrypt($row['auction_id'], $ciphering, $decryption_key, $options, $decryption_iv);
echo $bidder_id . " - " . $auction_id . "<br>";
        try {
            $conn_auction = new PDO("mysql:host=$servername;dbname=$dbname", $username, $password);
            // set the PDO error mode to exception
            $conn_auction->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
            $results_auction = $conn_auction->query('SELECT * FROM auctions');
            while ($row_auction = $results_auction->fetch()) {
                if ($row_auction['auctions_id'] == $auction_id) {
                    $item_auctionid[$mybidding_max] = $row_auction['auctions_id'];
                    $item_title[$mybidding_max] = openssl_decrypt($row_auction['title'], $ciphering, $decryption_key, $options, $decryption_iv);
                    $item_current_bid[$mybidding_max] = $row_auction['current_bid'];
$start_time = openssl_decrypt($row_auction['start_time'], $ciphering, $decryption_key, $options, $decryption_iv);
$duration = $row_auction['listing_days'];
//echo date('Y-m-d', strtotime($Date. ' + 10 days'));
$ending = date('Y-m-d H:i:s', strtotime($start_time. ' + $duration days'));
$item_time_left[$mybidding_max] = $ending;

                    $item_status[$mybidding_max] = openssl_decrypt($row_auction['status'], $ciphering, $decryption_key, $options, $decryption_iv);
                    echo  $item_auctionid[$mybidding_max] . " " . $item_title[$mybidding_max] . " " . $item_current_bid[$mybidding_max] . " " . $item_status[$mybidding_max] . "<br>";
                    $mybidding_max++;
                }
            }
        }catch(PDOException $e) {echo "A:-".$e;}
        $_SESSION['mybidding'] = $mybidding_max;
    }
}catch(PDOException $e) {echo "B:-".$e;}
?>

If the above works, how how can I show time remaining in real time?

Thanks.