vexxhost again has another good tutorial : How to convert/encode files to FLV using FFMPEG & PHP
from the site :
So, as I’ve written in an earlier article on how to install FFMPEG on your server, while there are those who probably use a “YouTube Clone” script, there might be those who want to create their own using FFMPEG & PHP. FLV is the most widely used type of codec that runs on most Flash players.
So, let’s get started, there are actually a few steps into converting a file to FLV which are shown below