Pilli

I have a form where I put in start time and end time eg $start= 11:00; $end=16:30; What i would like to do is get the total number of hours eg. total hours between 11:00 and 16:30 I tried $difference= $end-$start; echo "$difference"; but I get the answer 5 when it should be 5:30 I think I should be using the date function but not sure how to subtract two times. A point in the right direction would be most appreciated.