comet realization with ajax&php

1.prepare front-end code, meta content-type cannot be ignored! as to the xhr, status should be 3

<!doctype html>
</html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
</head>
<body>

    <div id="d"></div>

    <script type="text/javascript" src="/dwztp/Public/dwz/js/jquery-1.7.2.min.js"></script>
    <script type="text/javascript">
    var xhr=new XMLHttpRequest(),pos=0;
    xhr.onreadystatechange=function(){
        if(xhr.readyState==3){
            $("#d").html(xhr.responseText.substr(pos));
            pos= xhr.responseText.length;
        }
        else{
            $("#d").html(xhr.responseText);
        }
    };
    $.ajaxSetup({cache:false});
    xhr.open("get","a.php",true);
    xhr.send(null);
    // $.get("a.php",function(data){
    //     //document.write(data);
    // });
    </script>
</body>

</html>

2.back-end, remember to to call two methods to set time limit to unlimited and cache-control to be no-cache;

<?
    set_time_limit(0);
    header("Cache-Control: no-cache, must-revalidate");
    echo(date(‘Y‘));
    $no=0;
    while (date(‘Y‘) == ‘2016‘){
        echo ‘hello 2016+‘.$no.‘<br/>‘;
        ob_flush();
        flush();
        sleep(2);
        $no++;
    }
?>