Look at the PHP DateTime
object.
$dateA = new DateTime('2:00');
$dateB = new DateTime('3:00');
$difference = $dateA->diff($dateB);
(assuming you have >= PHP 5.3)
You can also do it the procedural way...
$dateA = strtotime('2:00');
$dateB = strtotime('3:00');
$difference = $dateB - $dateA;
See it on CodePad.org.
You can get the hour offset like so...
$hours = $difference / 3600;
If you are dealing with times that fall between a 24 hour period (0:00 - 23:59), you could also do...
$hours = (int) date('g', $difference);
Though that is probably too inflexible to be worth implementing.
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