Who Knows How Web-based Chat Works? And How To Do It? like facebook chat

I like to know how to create a web-based chat ... like YM or in facebook chat box ... in PHP and javascript

Why don't you have a look through the tutorials section on the forums. I've written a shoutbox tutorial, shoutbox being the new name for most browser-based chatrooms after the banning of most online chatrooms. It uses PHP, MySQL and AJAX (Javascript).Basically the way it works is, obviously you can't have live php running on the page as it is a server-side script. This is where ajax comes in. Instead of having to continuously keep refreshing the page to view the latest content ajax calls a php file in to the current document. So index.php loads up once and the page stays the same, ajax keeps updating a div on the page with the content output of chat.php without index.php having to refresh.