The split (incidentally, [man]explode[/man] is preferred, and [man]split[/man] is to be deprecated) and mktime are unnecessary.
Take the difference between the two hours, multiply it by 60, and add it to the difference between the two minutes.
15:27 - 06:34 = (15- 6)*60+(27-34) = 9*60-7 = 533
10:28 - 12:06 = (10-12)*60+(28- 6) = -2*60+22 = -98
11:56 - 11:18 = (11-11)*60+(56-18) = 0*60+38 = 38
Which, incidentally, would avoid your getting different results at certain times of the year.