未捕获的SyntaxError：输入Chrome的意外结束

Question

I've to write an long poll query from my database - currently using mysqli. I'm successfully calling the data and return in my javascript code, but, now is the trick part, after 2 minutes, I receive this error: "Uncaught SyntaxError: Unexpected end of input".

I googled and read a couple pages (more then 30 I guess), but I wasn't able to solve this one... everyone says is most likely that I forgot to close some brackets... but I guess not..

In other hand I prefer use json.parse() than eval().. and neither of them is working...

Thank for the attention.
Ps.: I'm not english native speacker, sorry any misspelling;)

---

That is my current js file

var old_msg_id = "<?php echo $old_msg_id; ?>";

function waitForMsg() {
    $.ajax({
        type: "GET",
        url: "poll.php?old_msg_id=" + old_msg_id,
        async: true,
        cache: false,
        //dataType : 'json',

        success: function (dataRespond) {
            //var jsonAnswer = eval("(" + dataRespond + ")");
            var jsonAnswer = JSON.parse(dataRespond);
            if (jsonAnswer.msg !== "") {
                alert("New msg added to base!");
                console.log(jsonAnswer.msg);
            };
            old_msg_id = jsonAnswer.old_msg_id;
            setTimeout('waitForMsg()', 1000);
        },
        error: function (XMLHttpRequest, textStatus, errorThrown) {
            alert("Deu merda!: Error: " + textStatus + " (" + errorThrown + ")");
            setTimeout('waitForMsg()', 15000);
        }
    });
}
$(document).ready(function () {
    waitForMsg();
});

That is my poll.php

error_reporting(0);
$conn = mysql_connect("localhost", "root", "");
mysql_select_db('padova', $conn) or die('Could not select database.');
$result = mysql_query("SELECT id FROM test ORDER BY id DESC LIMIT 1");
if($result === FALSE) {
    die(mysql_error());
}
$old_msg_id = $_GET['old_msg_id']; 
$result = mysql_query("SELECT id, text FROM test ORDER BY id DESC LIMIT 1");
while($row = mysql_fetch_array($result))
{
    $last_msg_id = $row['id'];
    $msg = $row['text'];
}
while($last_msg_id <= $old_msg_id)
{
    usleep(1000);
    clearstatcache();
    $result = mysql_query("SELECT id, text FROM test ORDER BY id DESC LIMIT 1");
    while($row = mysql_fetch_array($result))
    {
        $last_msg_id = $row['id'];
        $msg = $row['text'];
    }
}
$response = array();
$response['msg'] = $msg;
$response['old_msg_id'] = $last_msg_id;
$response = array_map('htmlentities',$response);
echo json_encode($response);