简单的自动更新AJAX页面

所以我对AJAX有点陌生，我正在努力让它发挥作用。我想做的是创建一个消息应用程序，每3秒自动更新一次。

这是我的脚本：

function first() {
    var searchUser = $("input[name='username']").val();
    $.post("messageSearch.php", {userVal: searchUser}, function(output){
        $('#messageField').html(output);
    });
}
function searchm() {
    var searchUser = $("input[name='username']").val();
    $.post("messageSearch.php", {userVal: searchUser}, function(output){
        $('#messageField').val(output);
    });
}
setInterval( "searchm()", 3000 );

这是我的信息Search.php：

<?php
session_start();
$userdb = new mysqli('localhost', 'test', '', 'social-network');
if(isset($_POST['userVal'])) {
    $searchm = $_POST['userVal'];
    $output = '';
    if ($searchm == ''){
        echo $output;
        exit();
    }
    $uidquery = mysqli_query($userdb, "SELECT * FROM users WHERE username='$searchm' LIMIT 1");
    $uid= '';
    while($row2 = mysqli_fetch_array($uidquery)) {
        $uid = $row2['id'];
    }
    $uid = 2;
    $query = mysqli_query($userdb, "SELECT * FROM messages WHERE p2=$uid AND `read`='n' LIMIT 3");
    $count = mysqli_num_rows($query);
    if($count == 0) {
        $output = 'You have no messages.';
    } else {
        while($row = mysqli_fetch_array($query)) {
            $from = $row['p1'];
            $message = $row['message'];
            $time = $row['time'];
            $time = date('Y-m-d H:i:s', strtotime($time));
            $fromResult = mysqli_query($userdb, "SELECT * FROM users WHERE id = '$from'");
            while($row1 = mysqli_fetch_array($fromResult)) {
                $fromFirst = $row1['first_name'];
                $fromLast = $row1['last_name'];
                $from = $fromFirst.' '.$fromLast;
            }
            $output .= '
<li>
    <a href="#">
        <div>
            <strong>'.$fromFirst.' '.$fromLast.'</strong>
            <span class="pull-right text-muted">
                <em>'.$time.'</em>
            </span>
        </div>
        <div>'.$message.'</div>
    </a>
</li>
<li class="divider"></li>
            ';
        }
        $output .= '<li><a class="text-center" href="#"><strong>See All Messages</strong> <i class="fa fa-angle-right"></i></a></li>';
    }
}
echo ($output);
?>

由于某种原因，第一个负载运行良好，尽管第二个负载出现空白。希望你能帮忙，不过提前谢谢。