Subtract time in PHP

If you get valid date strings, you can use this:

$workingHours = (strtotime($end) - strtotime($start)) / 3600;

This will give you the hours a person has been working.

A bit nicer is the following:

$a = new DateTime('08:00');
$b = new DateTime('16:00');
$interval = $a->diff($b);

echo $interval->format("%H");

That will give you the difference in hours.