if isset SESSION显示div

How do i do in jquery, to check if there's anything in isset SESSION user_message, each 1 seconds, and if there is then display #box else dont.. thanks

Use PHP to echo the jQuery command:

if (isset($_SESSION['user_message'])) {
    echo '<script> $("#box").… </script>';
}

This will only echo the following if $_SESSION['user_message'] is set:

<script> $("#box").… </script>

Simple as this:

<?php
if (isset($_SESSION['user_message']))
{
?>

<script>
$('#box').show(2000);
</script>

<?php
}
?>

Or you can use fadeIn or fadeOut with specified time.

<?php if (isset($_SESSION['user_message']))  { ?>
    <script type="text/javasscript">
        $("#box").show(1000);
    </script>
<?php } ?>

PHP file (session.php):

<?php if( isset($_SESSION) ){echo 1;}?>

JS file:

function checkSession(){
    $.ajax({url: "session.php", success: function(data){
         if( data == 1){
             $('#box').show();
         }else{
             $('#box').hide();
         }
    }});
}
setInterval('checkSession()',1000);

This is untested, but it should do what you're looking for. Note that, typically, AJAX request expect XML or JSON, so you can reformat the session.php file the way you want.