Which technologies I need to learn for developing Online Chat Room with voice/video support? Presently I know Core PHP/MySql and DHTML.
If you want to do a chat that would scale, you need to learn how to write an event handled application, there is a lot of framework to build such application, a short list
This tool would permit you to use the comet style of ajax call.
Sadly, I don't think there is any event driven web server/framework for PHP, so you would probably have to learn other language.