Calculate total seconds in PHP DateInterval

Could you not compare the time stamps instead?

$now = new DateTime('now');
$diff = $date->getTimestamp() - $now->getTimestamp()

This function allows you to get the total duration in seconds from a DateInterval object

/**
 * @param DateInterval $dateInterval
 * @return int seconds
 */
function dateIntervalToSeconds($dateInterval)
{
    $reference = new DateTimeImmutable;
    $endTime = $reference->add($dateInterval);

    return $endTime->getTimestamp() - $reference->getTimestamp();
}