I'm a fan of the dateinterval myself. Hours is a pain though, so for that I'd still suggest sticking with strtotime (or massage the DateInterval which would be more work). It's a bit easier to use overall IMO.
$diDiff is the daterInterval difference between the two. Best used for large dates, not small (it doesn't keep track of a total number of hours, only a total number of days).
$time1= new DateTime("24-11-2010 11:6:57");
$time2= new DateTime("24-11-2010 19:4:35");
$diDiff = $time1->diff($time2);
The diff on dateTime also has an inversion parameter to always force absolute.
print('Difference is ' . $diDiff->format('%y years, %m months, %d days, %h hours (total days: %a)'));
So with the smaller times, I would recommend you use simple subtraction (either using strtotime to get the integer, or by using DateTime and pulling the integer out) as indicated here. For anything larger, I'd suggest the DateInterval (to see how many years, months, days, etc ago). The DateInterval doesn't play great with hours, but can be easily calculated based on the total number of days past.
And yes, I'm also aware that this thread is a couple of years old now, so the DateInterval would just *barely* have applied as it was released mid 2009.