Q: How to get the total hour from starting time to end time in php

Q: How to get the total hour from starting time to end time in php

how can i get the total hour from start time to end time.
$start_time = '11:00:00 PM'; // as of 07/08/2013
$end_time = '01:00:00 AM'; // as of 08/08/2013
Then output should be:
$total = '02:00:00'; // 12 hour format
Regards!.