How to make a chat room script with PHP?

Question

Several visitors connect to http://site.com/chat.php

They each can write and send a text message to chat.php and it displays instantly on everyone's browser (http://site.com/chat.php)

Do I have to use a database? I mean, is AJAX or PHP buffer capabilities enough for such a chat room on sessions?

How can sessions of different users share data from each other?

Any idea or insights will be appreciated, thanks!

Edit: Thanks for the links. But what I want is the way to push data to a client browser. Is constantly refreshing client browser (AJAX or not) the only way? Also the challenge here is how different users, for example, 2, 1 on 1, share chat texts? How do you store them? And how do you synchronize the texts between the 2 clients? Not using a database preferably.

Edit 2: Actually YShout mentioned by Peter D does this job pretty well. It doesn't seem to keep refresh the browser. But I don't understand how it pushes new messages to existing user's window.

Answer 1

there are (roughly) 3 options for creating a chat application:

sockets

use flash/java and sockets for the frontend and a socket-capable programming language for the backend. for the backend, i'd recommend java or python, because they are multithreading and NIO-capable. it's possible to do it with PHP (but php can't really do efficient multithreading and is generally not really suited for this). this is an option if you need high performance, and probably not what you're looking for.

use ajax and pull

in this case all clients are constantly (for example ever 2 seconds) polling if something new has happened. it feels strange because you only get responses at those intervals. additionally, it puts quite a strain on your server and bandwidth. you know an application uses this technique because the browser constantly refreshes. this is a suboptimal solution.

use ajax and push

this works with multipart-responses and has long running (php-) scripts in the backend. not the best solution, but most of the time it's better than pulling and it works and is used in several well known chat apps. this technique is sometimes called COMET.

my advise: if you need a chat app for production use, install an existing one. programming chat applications is not that easy.

if you just want to learn it, start with a simple ajax/pull app, then try to program one using ajax and push.

and yes, most probably you'll need a database, tough i successfully implemented a very simple ajax/pull solution that works with text files for fun (but i certainly wouldn't use it in production!).

it is (to my knowledge, but i'm pretty sure) not possible to create a chat app without a server-side backend (with just frontend javascript alone)!

UPDATE

if you want to know how the data pushing is done, look at the source here: http://wehrlos.strain.at/httpreq/client.html. async multipart is what you want :)

function asSendSyncMulti() {
    var httpReq = new XMLHttpRequest();

    showMessage( 'Sending Sync Multipart ' + (++this.reqCount)  );

    // Sync - wait until data arrives
    httpReq.multipart   = true;     
    httpReq.open( 'GET', 'server.php?multipart=true&c=' + (this.reqCount), false );
    httpReq.onload = showReq;
    httpReq.send( null );
}

function showReq( event ) {
    if ( event.target.readyState == 4 ) {
        showMessage( 'Data arrives: ' + event.target.responseText );
    }
    else {
        alert( 'an error occured: ' + event.target.readyState );
    }

}

showReq is called every time data arrives, not just once like in regular ajax-requests (i'm not using jquery or prototype here, so the code's a bit obese - this is really old :)).

here's the server side part:

<?php

    $c = $_GET[ 'c' ];

    header('Content-type: multipart/x-mixed-replace;boundary="rn9012"');

    sleep( 1 );

    print "--rn9012\n";
    print "Content-type: application/xml\n\n";
    print "\n";
    print "Multipart: First Part of Request " . $c . "\n";
    print "--rn9012\n";
    flush();

    sleep( 3 );

    print "Content-type: application/xml\n\n";
    print "\n";
    print "Multipart: Second Part of Request " . $c . "\n";
    print "--rn9012--\n";

?>

update2

regarding the database: if you've got a nothing-shared architecture like mod_php/cgi in the backend, you definitley need some kind of external storage like databases or textfiles. but: you could rely on memory by writing your own http server (possible with php, but i'd not recommend it for serious work). that's not really complicated, but probably a bit out of the scope of your question ^^

update3

i made a mistake! got everything mixed up, because it's been a long time i actually did something like that. here are the corrections:

1. multipart responses only work with mozilla browsers and therefore are of limited use. COMET doesn't mean multipart-response.

2. COMET means: traditional singlepart response, but held (with an infinite loop and sleep) until there is data available. so the browser has 1 request/response for every action (in the worst case), not one request every x seconds, even if nothing response-worthy happens.