This is a fast post, but if you are trying to get a FLV or MPEG duration with PHP and don’t know how, this is a great toolkit to have in hands.
Besides the getID3 library, you do not need to install anything. It will mimic very well the FF-mpeg library for PHP.
You can get the getID3 libary at: http://getid3.sourceforge.net/
Great resource if you are thinking about handling videos, capturing a snapshot of one, etc.