无法通过jquery将变量传递到下一页

I'm making a chat system between users and operator,which contain of chatbox written by html and posechat.php to bring the data from database to the chatbox via jquery.

<div id="chatbox"></div>

    <form>
        <input name="usermsg" type="text" id="usermsg" size="63" />
        <input type="hidden" id="uid" name="uid" value="<?php echo $uid; ?>">
        <input name="submitmsg" type="submit"  id="submitmsg" value="Send" />
    </form>

jquery:

 $("#submitmsg").click(function () {
            var clientmsg = $("#usermsg").val();
            var clientid = $("#uid").val();
            $.post("includes/postchat.php", {text: clientmsg,uid: clientid});
            $.post("includes/posechat.php", {text: clientmsg,uid: clientid});
            $("#usermsg").attr("value", "");
            return false;
        });
$.ajaxSetup({cache: false});

        setInterval(function () {
            $('#chatbox').load('includes/posechat.php');
        }, 1000);
    });

I put an username in the tail of chatbox to identify the the user ( ex: chatsystem.html?user=Adam).I want to fetch that data($uid) to the posechat.php for sql statement to display the right output in the chatbox but it doesnt seem to work, got an error: Undefined index Here's the posechat.php:

session_start();

$user = $_SESSION['u_uid'];

include('dbh-inc.php');
$uid = $_POST['uid'];
echo $uid;