difference of two dates in seconds php

	date_default_timezone_set("Africa/Johannesburg");
    $now = new DateTime();
    $future_date = new DateTime('2020-10-21 00:00:00');
    
    $interval = $future_date->diff($now);
    
    echo ($interval->format("%a") * 24) + $interval->format("%h"). " hours". $interval->format(" %i minutes ");
    print_r($now->format('Y-m-d H:i:s'));

$timeFirst  = strtotime('2011-05-12 18:20:20');
$timeSecond = strtotime('2011-05-13 18:20:20');
$differenceInSeconds = $timeSecond - $timeFirst;