Processing form values for each day of week
Well, every time you see a repeated code, you could think of a loop. 

if I am not mistaken, only a weekday is changed, so it could be used to form the loop:

$schedule = [];
$weekdays = ['mon','tue','wed','thu','fri','sat','sun'];
foreach ($weekdays as $day)
{
    $schedule[] = round(abs(strtotime($_POST[$day][1]) - strtotime($_POST[$day][0])) / 3600, 2),
    $schedule[] = round(abs(strtotime($_POST[$day][3]) - strtotime($_POST[$day][2])) / 3600, 2),
}