PHP + jQuery + MySQL async fetching data, script ends unexpected - php

Question

I'm trying to use jQuery's $.ajax() to GET a PHP page, on a based interval, which prints the latest enteries from a MySQL table. If there are new enteries, they are printed on the page, if not, "NaN" gets printed.
This is my PHP function which checks for new enteries: http://pastebin.com/R6NHDU3t
This is the HTML + jQuery which checks for response from the above function(got it from some answer here on Stack): http://pastebin.com/myT7evAx
The idea is this: I access the HTML page, the jQuery inside is GET-ing the PHP page every 3 seconds, the PHP checkes for new enteries inside MySQL; if there are new enteries, they are printed on the PHP page and get fetched by jQuery, then appended into #messages, if not, "NaN" is printed(because of JSON, doesen't matter).
This works fine if no new enteries are available: every 3 seconds "NaN" gets printed on the page; some Live HTTP Headers:
http://localhost/handler.class.php?Q=comments&_=1348715334458
Host: localhost
User-Agent: Mozilla/5.0 (Windows NT 5.1; rv:14.0) Gecko/20100101 Firefox/14.0.1
Accept: */*
Accept-Language: en-us,en;q=0.5
Accept-Encoding: gzip, deflate
Connection: keep-alive
X-Requested-With: XMLHttpRequest
Referer: http://localhost/live.html
HTTP/1.1 200 OK
Date: Thu, 27 Sep 2012 03:08:54 GMT
Server: Apache/2.2.22 (Win32) PHP/5.3.13
X-Powered-By: PHP/5.3.13
Content-Length: 15 <---- the "NaN", no new data
Keep-Alive: timeout=5, max=97
Connection: Keep-Alive
Content-Type: text/html
If new enteries are made, the script stopps appending "NaN"(doh, new data), and it hould append my new enteries, but the script stopps, no new data is appended and it stopps making requests. Here is the latest Live HTTP Headers log, showing the last request:
http://localhost/handler.class.php?Q=comments&_=1348715337473
GET /handler.class.php?Q=comments&_=1348715337473 HTTP/1.1
Host: localhost
User-Agent: Mozilla/5.0 (Windows NT 5.1; rv:14.0) Gecko/20100101 Firefox/14.0.1
Accept: */*
Accept-Language: en-us,en;q=0.5
Accept-Encoding: gzip, deflate
Connection: keep-alive
X-Requested-With: XMLHttpRequest
Referer: http://localhost/live.html
HTTP/1.1 200 OK
Date: Thu, 27 Sep 2012 03:08:57 GMT
Server: Apache/2.2.22 (Win32) PHP/5.3.13
X-Powered-By: PHP/5.3.13
Content-Length: 3257 <-- new data, so it got the response, but stopped here. no appending, no more requests.
Keep-Alive: timeout=5, max=96
Connection: Keep-Alive
Content-Type: text/html
Why does it stop when handler.class.php?Q=comments returns new data?