convertion from string to time & substracting

Hi, sorry if maybe this is a bit dumb question, but I'm a begginer with php... I got a mysql database, where two of the fields of a table record times as CHAR(8) in the format hh:mm:ss I want to take this two times and get the difference between them in seconds, for example 12:01:30 - 12:00:00 = 90 I looked up at the doc in the php.net website and couldn't get on how to do this... should I use the strtotime function? anyone has donde something like this, or knows what functions could help me to do this? Thanks in advance, juaid