Among other things, this makes the business of compiling web metrics more complex: a Gmail user can send and receive hundreds of email messages while staying on the same page.
When it's complete, you will be able to choose among users who are on your buddy list and send and receive messages through the Web page to users on traditional IM clients.
PHP is mainly focused on server-side scripting, so you can do anything any other CGI program can do, such as collect form data, generate dynamic page content, or send and receive cookies.