仅在有新数据时重新加载div

If I create a script like this, then it will reload the div every 2.5 seconds. I want a script that only displays if there is new data, if there is no new data it does not have to reload...

<script type="text/javascript">
    function dispMsg() {
        $("#displayMessage").load('load.php');
        var newscrollHeight = $("#displayMessage").attr("scrollHeight") - 20;
        $("#displayMessage").animate({ scrollTop: newscrollHeight }, 'normal');
    }
    setInterval (dispMsg, 2500);
});
</script>

<div id="displayMessage"></div>

and here is the load.php:

$sql = "SELECT * FROM message ORDER BY id DESC LIMIT 1";
$query = mysql_query($sql);
while ($result = mysql_fetch_array($query)) {
    $id = $result['id'];
    $from = $result['user_01'];
    $to = $result['to_usr'];
    $message = $result['message_01'];
    $date = $result['date_send'];

    echo "<span class='from'> $from </span>" 
      . "<span class='message'> $message </span> <br/>";
}

use the parameters and call back of $.load()

$.load(url, {param1:value, param2:value}, function(result){ 
    if(result >5){ 
     //do something
    }
)

Example:

<?php

if($_REQUEST['action'] == 'check'){
    if($_REQUEST['lastId'] < 5 ){
        echo $_REQUEST['lastId']+1;
        die;
    }
}

if($_REQUEST['action'] == 'load'){
    echo 'some conntent!';
    die;
}

?>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.7.1/jquery.min.js"></script>    
<script type="text/javascript">
$(document).ready(function(){
    var lastId = 0;
    function dispMsg() { 
        $.get('tmp.php', {action:'check', lastId: lastId}, function(responce){

            if(responce !== 0){
                alert(responce);
               $('#displayMessage').load('tmp.php', {action:'load'});
            }
        });

    }
    setInterval (dispMsg, 2500);
});
</script>

<div id="displayMessage">1</div>

You can send the last message id with each request. If the last message of your query is greater than the id sent, send the response. Otherwise send nothing.

function load_messages(){
    var last = $('#messagewindow p:last').attr('id');
    $.ajax({
        url: base_url + "load.php",
        type: "POST",
    data: {
        last: last
    },
        cache: false,
        success: function(html){
          //insert message
        }
    });
}

You can send the id as the id attribute in your HTML. Each message in the chat window is sent as a paragraph with some message id such as id="133".

Expanding on @Pé de Leão's solution, here's something more simple:

<?php
$sql = "SELECT * FROM message ORDER BY id DESC LIMIT 1";
$mysqli = new mysqli("host", "user", "pass");
if ($result = $mysqli->query($sql)) {
    $msg = $result->fetch_assoc();

    if (isset($_REQUEST['last']) && $msg['id'] != $_REQUEST['last']) {
        echo "<span id='${msg['id']}' class='from'>${msg['user_01']}</span>";
    }
}

And on the client side:

function dispMsg() {
    var last = $('#displayMessages span.from').attr('id');
    $.get(baseUrl + '/load.php', { last: last }, function(result) {
         if (result) {
             $('#displayMessages').html(result);
         }
    });
}

Note that this replaces everything in displayMessages.