<?php
if (condition) {
    // update new message to table
    $data['chatid'] = $to_id; // use a unique random string
    // broadcast using pusher notication
    // each active browser with pusher script will get the notification
    $pusher->trigger('__subscribed_channel__', '__chat_session__', $data);
}

?>

<!-- for user notification -->
<script type="text/javascript">

    // Enable pusher logging - don't include this in production
    Pusher.log = function(message) {
        if (window.console && window.console.log) {
            window.console.log(message);
        }
    };

    var pusher = new Pusher('__app_secret__');
    var channel = pusher.subscribe('__subscribed_channel__');
    channel.bind('__chat_session__', function(data) {
        // if broadcase notification received
        var element = document.getElementById("new_msg");
        // database was updated with your id
        // if your id is data.chatid goto server and check your messages
        if (data.chatid == '<?= $_SESSION['user']['chatid']; ?>') {
            // use ajax to get new messages so you can authenticate user and only result
            // you can create a alert with html/css
            // using push notication, you don't have to pull message with setInterval
            // if you use pulling, server will have to make query every interval EACH user
            // Hence, if you have 1000 active browser with 1 second interval, you may have 1000 query per second even when no new message is added to server
        } else {
        // do nothing
        }
    });
</script>