Subtract timestamp time from current time

I want to subtract database timestamp from current server time. The formatted time that is returned to me from my database is "2013-09-01 22:05:39" and I use function date("Y-m-d H:i:s"); so that my server time I get in php is in the same format.

How can I get a result in hours? Example: 2013-09-01 22:05:39  minus 2013-09-02 22:05:39 equals 24 hours ???