dataTables bottstrap issues with pagination

I copied example from site too test how it works etc. But i'm having issues with pagination.
It's not working.

http://invision.hr/dataTables/tables.html

Here is the live example.
And here is my php implementation

$columns_num = 5;

$offset = (empty($_GET['iDisplayStart'])) ? 0 : $_GET['iDisplayStart'];
$limit = (empty($_GET['iDisplayLength'])) ? 10 : $_GET['iDisplayLength'];

$show_total = 200;
$return_data = array();

function getrandomstring($length) {
       global $template;
       settype($template, "string");

       $template = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
       /* this line can include numbers or not */

       settype($length, "integer");
       settype($rndstring, "string");
       settype($a, "integer");
       settype($b, "integer");

       for ($a = 0; $a <= $length; $a++) {
               $b = rand(0, strlen($template) - 1);
               $rndstring .= $template[$b];
       }

       return $rndstring; 
}


for ($i = $offset; $i <= ($offset+$limit); $i++) {
    $data = array();
    $data[] = $i;
    for ($a = 1; $a < $columns_num; $a++) {
        $data[] = getrandomstring(6);
    }

    $return_data['aaData'][] = $data;
}

$return_data["sEcho"] = 1;
$return_data["iTotalRecords"] = $show_total;
$return_data["iTotalDisplayRecords"] = $show_total;


echo json_encode($return_data);exit;

I'm just sending random data in array...it's straightforward just for this test.

What am i doing wrong?