This function calculates the difference in hours between 2 complete periods.

calculateDifferenceBetweenTwoPeriods

function calculateDifferenceBetweenTwoPeriods($start, $end) {
    $start = new DateTime($start);
    $end = new DateTime($end);

    $interval = $start->diff($end);
	
	//Uncomment the line below to see all the returned values for the difference.
	//print_r($interval);

    $hour = str_pad($interval->h, 2, '0', STR_PAD_LEFT);
    $minute = str_pad($interval->i, 2, '0', STR_PAD_LEFT);
    $second = str_pad($interval->s, 2, '0', STR_PAD_LEFT);

    return $hour.':'.$minute.':'.$second;
}

$start = '2021-05-01 20:20:25';
$end = '2021-05-02 03:05:15';
$difference = calculateDifferenceBetweenPeriods($start, $end);

/*
The return is 06:44:50 hours.
*/