Well, since your time only has a specific clock time no day or months, you can explode the time and convert it into seconds. Like this:
$tme = explode(":", $b2);
$secs = ($tme * 3600) + $tme * 60 + $tme;
Do the same for the second time then subtract them. What you will have is the time difference in seconds. You can do an if > 60 to convert into minutes, if > 3600 to convert to hours.
And also try to format your PHP code.