To develop a chat

the Required minimum functionality:

Front-end client
- Visual state of the chat window
-- Rolled
-- Banner
-- Detailed
-- In a separate window
-- Offline
- Audible and visual alert when a message is received
- Ability to send files

Back end
Authorization
- Html editor visual States of the chat window on the front-end
Navigation
-- A list of current chats
-- Chat history
-- Offline messages

implementation serverside php websocket(probably comet).
You deadlines and the approximate cost