What is Event Driven Programming, its uses, & how to implement it in PHP?

That is a question that came up recently in a PHP group on Linkedin. I thought
it might also be worth discussing here, too.

Some years ago I became acquainted with a rather robust system for managing
credit unions where the entire UI was implemented through a single command line
prompt, like a Unix or DOS shell. Quite literally, the only two events that the
server would respond to were the Enter and Escape keys. Interaction with the
server would begin by entering a short action code, which would generally
trigger a dialog to enter data into a database, perform transactions, and so
forth, one input line at a time.

Perhaps surprisingly, users liked it, and became productive with it, but
contrast that type of UI with an HTML page that may be filled with any number of
visual and input capable elements simultaneously, and using
element.addEventListener() or element.attachEvent() to register a potentially
broad range of keyboard and mouse event handlers for any number UI element
displayed.

I suppose you could carry the comparison (contrast) even further by using AJAX
to subscribe to a network service which may be "listening" for events on a
network, and notify each client of it. A good use case might be a moderated chat
service. It might be helpful to update each user's screen whenever a participant
on the network enters or leaves the chat service. Maybe you want their name
begin flashing when they begin typing. Every client should be notified the
moment any one of them enters a new message.

I'm not sure how to implement something that in PHP. I assume it can be done
because Facebook chat works something like that. Using a tool like Fiddler, you
can see the browser performing an asynchronous request that waits for up to
something like 2 minutes, for the server respond. I assume the server in turn is
waiting on a queue of sorts. But I'm not aware of the implementation details on
the server, beyond configuring the HTTP server to allow persistent connections.


Contrast network listening, vs network polling. Polling from the client at
regular intervals consumes more bandwidth and other resources, and yet is not
quite as responsive as listening on a queue.

-Nathan