Converting MP4 to OGG and WEBM formats with PHP and FFMPEG

Converting MP4 to OGG and WEBM formats with PHP and FFMPEG

I want to write my own video sharing website (php). Different browsers
support different file formats. I need only 3 formats - OGG, MP4, and
WEBM. Upload will support only MP4 files. So I have a problem about video
converting. I dont know how to use ffmpeg. How can I convert the mp4 file
to ogg and webm with ffmpeg and php. Please, answer from 0. (Sorry, for my
poor English) Thanks :)