PHP/JQuery - Send data to a client from another client - php

Question

I'm building a person to person chat, and want person A's page to refresh, loading new messages from Person B when Person B sends them. How would I send a message/data to Person A when Person B sends a message via PHP? I know I can check on Person A's page via Ajax, but constantly running a MySQL query would drastically bring down the server's speed. Any ideas?
EDIT: Using Server Sent Events, here's my script code:
if(typeof(EventSource) !== "undefined") {
var source = new EventSource("update.php?user=<? echo $recip ?>");
source.onmessage = function(event) {
document.write(event.data);
if (event.data=="yes"){
window.location.href="/chat?with=<? echo $recip ?>";
}
};
} else {
document.getElementById('info-text').innerHTML="Hmm... looks like your browser doesn't support auto updating. Please refresh the page to check for new messages." //'
}
And here's my PHP code:
header('Content-Type: text/event-stream');
header('Cache-Control: no-cache');
$user=$_GET['user'];
$sql=mysql_query("SELECT * FROM chatmsg WHERE sender='$myusername' AND receiver='$recip' OR sender='$recip' AND receiver='$myusername'");
$newrows=mysql_num_rows($sql);
if ($newrows!=$_SESSION['chat'.$user]) {
echo "data: yes";
flush();
}
else {
echo "data: no";
flush();
The problem is, nothing is happening when there's a new row in MySQL.

Answer 1

I found the solution, everyone. I still used the Server Sent Events, but made some changes and found the error. Here's the final working code:
PHP:
header('Content-Type: text/event-stream');
header('Cache-Control: no-cache');
header("Access-Control-Allow-Origin: *");
$user=$_GET['user'];
$sql=mysql_query("SELECT * FROM chatmsg WHERE sender='$myusername' AND receiver='$user' OR sender='$user' AND receiver='$myusername'");
$newrows=mysql_num_rows($sql);
if ($newrows!==$_SESSION['chat'.$user]) {
$msg="yes";
}
else {
$msg="no";
}
echo "data: {$msg}\n\n";
flush();
sleep(10);
(sleep is to save server resources).
JS:
var source = new EventSource('update.php?user=<? echo $recip ?>');
source.onmessage = function(e) {
if (e.data=="yes") {
window.location.href="viewchat.php?viewer=<? echo $viewer ?>&recip=<? echo $recip ?>";
}
}